a2p
accept
access
acct
addftinfo
addr2line
adjtime
afmtodit
after
aio_cancel
aio_error
aio_read
aio_return
aio_suspend
aio_waitcomplete
aio_write
alias
aliases
alloc
anvil
append
apply
apropos
ar
array
as
asa
asn1parse
at
atq
atrm
attemptckalloc
attemptckrealloc
authlib
authtest
autopoint
awk
b64decode
b64encode
basename
batch
bc
bdes
bell
bg
bgerror
biff
big5
binary
bind
bindkey
bindtags
bindtextdomain
bio
bitmap
blowfish
bn
bootparams
bootptab
bounce
brandelf
break
breaksw
brk
bsdiff
bsdtar
bsnmpd
bspatch
bthost
btsockstat
buffer
builtin
builtins
bunzip2
button
byacc
bzcat
bzegrep
bzfgrep
bzgrep
bzip2
c2ph
c89
c99
ca
cal
calendar
canvas
cap_mkdb
case
cat
catch
catman
cc
cd
cdcontrol
chdir
checkbutton
checknr
chflags
chfn
chgrp
chio
chkey
chmod
chown
chpass
chroot
chsh
ci
ciphers
ckalloc
ckdist
ckfree
ckrealloc
cksum
cleanup
clear
clipboard
clock
clock_getres
clock_gettime
clock_settime
close
cmp
co
col
colcrt
colldef
colors
colrm
column
comm
command
compile_et
complete
compress
concat
config
connect
console
continue
core
courierlogger
couriertcpd
cp
cpan
cpio
cpp
creat
crl
crontab
crunchgen
crunchide
crypt
crypto
csh
csplit
ctags
ctm
ctm_dequeue
ctm_rmail
ctm_smail
cu
cursor
cursors
cut
cvs
date
dbiprof
dbiproxy
dc
dcgettext
dcngettext
dd
dde
default
defer
deliverquota
des
destroy
devfs
df
dgettext
dgst
dh
dhparam
dialog
diff
diff3
dig
dir
dirent
dirname
dirs
discard
disktab
dngettext
do
domainname
done
dprofpp
dsa
dsaparam
dtmfdecode
du
dup
dup2
eaccess
ec
ecdsa
echo
echotc
ecparam
ed
edit
editrc
ee
egrep
elf
elfdump
elif
else
enc
enc2xs
encoding
end
endif
endsw
engine
enigma
entry
env
envsubst
eof
eqn
err
errno
error
errstr
esac
ethers
euc
eui64
eval
event
evp
ex
exec
execve
exit
expand
export
exports
expr
extattr
extattr_delete_fd
extattr_delete_file
extattr_get_fd
extattr_get_file
extattr_set_fd
extattr_set_file
f77
false
famm
famx
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fbtab
fc
fchdir
fchflags
fchmod
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fcntl
fconfigure
fcopy
fdescfs
fdformat
fdread
fdwrite
fetch
fg
fgrep
fhopen
fhstat
fhstatfs
fi
file
file2c
fileevent
filename
filetest
find
find2perl
finger
flex
flock
flush
fmt
focus
fold
font
fontedit
for
foreach
fork
format
forward
fpathconf
frame
from
fs
fstab
fstat
fstatfs
fsync
ftp
ftpchroot
ftpusers
ftruncate
futimes
g711conv
gb2312
gb18030
gbk
gcc
gcore
gcov
gdb
gencat
gendsa
genrsa
gensnmptree
getconf
getdents
getdirentries
getdtablesize
getegid
geteuid
getfacl
getfh
getfsstat
getgid
getgroups
getitimer
getlogin
getopt
getopts
getpeername
getpgid
getpgrp
getpid
getppid
getpriority
getresgid
getresuid
getrlimit
getrusage
gets
getsid
getsockname
getsockopt
gettext
gettextize
gettimeofday
gettytab
getuid
glob
global
gmake
goto
gperf
gprof
grab
grep
grid
grn
grodvi
groff
groff_font
groff_out
groff_tmac
grog
grolbp
grolj4
grops
grotty
group
groups
gunzip
gzcat
gzexe
gzip
h2ph
h2xs
hash
hashstat
hd
head
help2man
hesinfo
hexdump
history
host
hostname
hosts
hosts_access
hosts_options
hpftodit
http
hup
i386_get_ioperm
i386_get_ldt
i386_set_ioperm
i386_set_ldt
i386_vm86
iconv
id
ident
idprio
if
ifnames253
ifnames259
image
imapd
incr
indent
indxbib
info
infokey
inode
install
instmodsh
interp
intro
introduction
ioctl
ipcrm
ipcs
ipf
ipftest
ipnat
ippool
ipresend
issetugid
jail
jail_attach
jobid
jobs
join
jot
kbdcontrol
kbdmap
kcon
kdestroy
kdump
kenv
kevent
keycap
keylogin
keylogout
keymap
keysyms
kgdb
kill
killall
killpg
kinit
kldfind
kldfirstmod
kldload
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kqueue
kse
kse_create
kse_exit
kse_release
kse_switchin
kse_thr_interrupt
kse_wakeup
ktrace
label
labelframe
lam
lappend
last
lastcomm
lastlog
lchflags
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ld
ldap
ldapadd
ldapcompare
ldapdelete
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ldapmodrdn
ldappasswd
ldapsearch
ldapwhoami
ldd
leave
less
lesskey
lex
lgetfh
lhash
libnetcfg
library
limit
limits
lindex
link
linprocfs
linsert
lint
lio_listio
list
listbox
listen
lj4_font
lkbib
llength
lmtp
ln
load
loadfont
local
locale
locate
lock
lockf
log
logger
login
logins
logname
logout
look
lookbib
lorder
lower
lp
lpq
lpr
lprm
lptest
lrange
lreplace
ls
lsearch
lseek
lset
lsort
lstat
lsvfs
lutimes
lynx
m4
madvise
magic
mail
maildiracl
maildirkw
maildirmake
mailq
mailx
make
makeinfo
makewhatis
man
manpath
master
mc
mcedit
mcview
md2
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mdc2
memory
menu
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mesg
message
mincore
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minigzip
mkdep
mkdir
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mkstr
mktemp
mlock
mlockall
mmap
mmroff
modfind
modfnext
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modstat
moduli
more
motd
mount
mprotect
mptable
msdos
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msgattrib
msgcat
msgcmp
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msgconv
msgen
msgexec
msgfilter
msgfmt
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msginit
msgmerge
msgs
msgunfmt
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mt
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munlockall
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mv
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namespace
nanosleep
nawk
nc
ncal
ncplist
ncplogin
ncplogout
neqn
netconfig
netgroup
netid
netstat
networks
newaliases
newgrp
nex
nfsstat
nfssvc
ngettext
nice
nl
nm
nmount
nohup
nologin
notify
nroff
nseq
nslookup
ntp_adjtime
ntp_gettime
nvi
nview
objcopy
objdump
objformat
ocsp
od
onintr
open
openssl
opieaccess
opieinfo
opiekey
opiekeys
opiepasswd
option
options
oqmgr
pack
package
packagens
pagesize
palette
pam_auth
panedwindow
parray
passwd
paste
patch
pathchk
pathconf
pawd
pax
pbm
pcre
pcreapi
pcrebuild
pcrecallout
pcrecompat
pcrecpp
pcregrep
pcrematching
pcrepartial
pcrepattern
pcreperform
pcreposix
pcreprecompile
pcresample
pcretest
perl
perl56delta
perl58delta
perl561delta
perl570delta
perl571delta
perl572delta
perl573delta
perl581delta
perl582delta
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perl584delta
perl585delta
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perl587delta
perl588delta
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rcp
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ree
refer
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rename
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req
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resolver
resource
return
rev
revoke
rfcomm_sppd
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rhosts
ripemd
ripemd160
rlog
rlogin
rm
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rmdir
rpc
rpcgen
rs
rsa
rsautl
rsh
rtld
rtprio
rup
ruptime
rusers
rwall
rwho
s2p
safe
sasl
sasldblistusers2
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sbrk
scache
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scon
scp
script
scrollbar
sdiff
sed
seek
select
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semctl
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send
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services
sess_id
set
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setitimer
setlogin
setpgid
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setpriority
setregid
setresgid
setresuid
setreuid
setrlimit
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settc
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sh
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sigreturn
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sigstack
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size
slapadd
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slapd
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slapindex
slappasswd
slaptest
sleep
slogin
slurpd
smbutil
smime
smtp
smtpd
socket
socketpair
sockstat
soelim
sort
source
spawn
speed
spinbox
spkac
splain
split
squid
squid_ldap_auth
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ssh
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ssh_config
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startslip
stat
statfs
stop
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strings
strip
stty
su
subst
sum
suspend
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swapon
switch
symlink
sync
sysarch
syscall
sysconftool
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s_client
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tabs
tail
talk
tar
tbl
tclsh
tcltest
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tcopy
tcpdump
tcpslice
tcsh
tee
tell
telltc
telnet
term
termcap
terminfo
test
texindex
texinfo
text
textdomain
tfmtodit
tftp
then
threads
time
tip
tk
tkerror
tkvars
tkwait
tlsmgr
tmac
top
toplevel
touch
tput
tr
trace
trafshow
trap
troff
true
truncate
truss
tset
tsort
tty
ttys
type
tzfile
ui
ul
ulimit
umask
unalias
uname
uncomplete
uncompress
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unhash
unifdef
unifdefall
uniq
units
unknown
unlimit
unlink
unmount
unset
unsetenv
until
unvis
update
uplevel
uptime
upvar
usbhidaction
usbhidctl
users
utf8
utimes
utmp
utrace
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uuencode
uuidgen
vacation
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verify
version
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vi
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view
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vis
vt220keys
vwait
w
wait
wait3
wait4
waitpid
wall
wc
wget
what
whatis
where
whereis
which
while
who
whoami
whois
window
winfo
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wm
write
writev
wtmp
x509
xargs
xgettext
xmlwf
xstr
xsubpp
yacc
yes
ypcat
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ypchpass
ypchsh
ypmatch
yppasswd
ypwhich
yyfix
zcat
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zdiff
zegrep
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perlfaq7
 
PERLFAQ7(1)	       Perl Programmers Reference Guide 	   PERLFAQ7(1)



NAME
       perlfaq7 - General Perl Language Issues ($Revision: 1.28 $, $Date:
       2005/12/31 00:54:37 $)

DESCRIPTION
       This section deals with general Perl language issues that don't clearly
       fit into any of the other sections.

       Can I get a BNF/yacc/RE for the Perl language?

       There is no BNF, but you can paw your way through the yacc grammar in
       perly.y in the source distribution if you're particularly brave.  The
       grammar relies on very smart tokenizing code, so be prepared to venture
       into toke.c as well.

       In the words of Chaim Frenkel: "Perl's grammar can not be reduced to
       BNF.  The work of parsing perl is distributed between yacc, the lexer,
       smoke and mirrors."

       What are all these $@%&* punctuation signs, and how do I know when to
       use them?

       They are type specifiers, as detailed in perldata:

	   $ for scalar values (number, string or reference)
	   @ for arrays
	   % for hashes (associative arrays)
	   & for subroutines (aka functions, procedures, methods)
	   * for all types of that symbol name.  In version 4 you used them like
	     pointers, but in modern perls you can just use references.

       There are couple of other symbols that you're likely to encounter that
       aren't really type specifiers:

	   <> are used for inputting a record from a filehandle.
	   \  takes a reference to something.

       Note that  is neither the type specifier for files nor the name
       of the handle.  It is the "<>" operator applied to the handle FILE.  It
       reads one line (well, record--see "$/" in perlvar) from the handle FILE
       in scalar context, or all lines in list context.  When performing open,
       close, or any other operation besides "<>" on files, or even when talk-
       ing about the handle, do not use the brackets.  These are correct:
       "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN to FILE".

       Do I always/never have to quote my strings or use semicolons and com-
       mas?

       Normally, a bareword doesn't need to be quoted, but in most cases prob-
       ably should be (and must be under "use strict").  But a hash key con-
       sisting of a simple word (that isn't the name of a defined subroutine)
       and the left-hand operand to the "=>" operator both count as though
       they were quoted:

	   This 		   is like this
	   ------------ 	   ---------------
	   $foo{line}		   $foo{'line'}
	   bar => stuff 	   'bar' => stuff

       The final semicolon in a block is optional, as is the final comma in a
       list.  Good style (see perlstyle) says to put them in except for
       one-liners:

	   if ($whoops) { exit 1 }
	   @nums = (1, 2, 3);

	   if ($whoops) {
	       exit 1;
	   }
	   @lines = (
	       "There Beren came from mountains cold",
	       "And lost he wandered under leaves",
	   );

       How do I skip some return values?

       One way is to treat the return values as a list and index into it:

	       $dir = (getpwnam($user))[7];

       Another way is to use undef as an element on the left-hand-side:

	   ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

       You can also use a list slice to select only the elements that you
       need:

	       ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];

       How do I temporarily block warnings?

       If you are running Perl 5.6.0 or better, the "use warnings" pragma
       allows fine control of what warning are produced.  See perllexwarn for
       more details.

	   {
	       no warnings;	     # temporarily turn off warnings
	       $a = $b + $c;	     # I know these might be undef
	   }

       Additionally, you can enable and disable categories of warnings.  You
       turn off the categories you want to ignore and you can still get other
       categories of warnings.	See perllexwarn for the complete details,
       including the category names and hierarchy.

	       {
	       no warnings 'uninitialized';
	       $a = $b + $c;
	       }

       If you have an older version of Perl, the $^W variable (documented in
       perlvar) controls runtime warnings for a block:

	   {
	       local $^W = 0;	     # temporarily turn off warnings
	       $a = $b + $c;	     # I know these might be undef
	   }

       Note that like all the punctuation variables, you cannot currently use
       my() on $^W, only local().

       What's an extension?

       An extension is a way of calling compiled C code from Perl.  Reading
       perlxstut is a good place to learn more about extensions.

       Why do Perl operators have different precedence than C operators?

       Actually, they don't.  All C operators that Perl copies have the same
       precedence in Perl as they do in C.  The problem is with operators that
       C doesn't have, especially functions that give a list context to every-
       thing on their right, eg. print, chmod, exec, and so on.  Such func-
       tions are called "list operators" and appear as such in the precedence
       table in perlop.

       A common mistake is to write:

	   unlink $file || die "snafu";

       This gets interpreted as:

	   unlink ($file || die "snafu");

       To avoid this problem, either put in extra parentheses or use the super
       low precedence "or" operator:

	   (unlink $file) || die "snafu";
	   unlink $file or die "snafu";

       The "English" operators ("and", "or", "xor", and "not") deliberately
       have precedence lower than that of list operators for just such situa-
       tions as the one above.

       Another operator with surprising precedence is exponentiation.  It
       binds more tightly even than unary minus, making "-2**2" product a neg-
       ative not a positive four.  It is also right-associating, meaning that
       "2**3**2" is two raised to the ninth power, not eight squared.

       Although it has the same precedence as in C, Perl's "?:" operator pro-
       duces an lvalue.  This assigns $x to either $a or $b, depending on the
       trueness of $maybe:

	   ($maybe ? $a : $b) = $x;

       How do I declare/create a structure?

       In general, you don't "declare" a structure.  Just use a (probably
       anonymous) hash reference.  See perlref and perldsc for details.
       Here's an example:

	   $person = {};		   # new anonymous hash
	   $person->{AGE}  = 24;	   # set field AGE to 24
	   $person->{NAME} = "Nat";	   # set field NAME to "Nat"

       If you're looking for something a bit more rigorous, try perltoot.

       How do I create a module?

       (contributed by brian d foy)

       perlmod, perlmodlib, perlmodstyle explain modules in all the gory
       details. perlnewmod gives a brief overview of the process along with a
       couple of suggestions about style.

       If you need to include C code or C library interfaces in your module,
       you'll need h2xs.  h2xs will create the module distribution structure
       and the initial interface files you'll need.  perlxs and perlxstut
       explain the details.

       If you don't need to use C code, other tools such as ExtUtils::Module-
       Maker and Module::Starter, can help you create a skeleton module dis-
       tribution.

       You may also want to see Sam Tregar's "Writing Perl Modules for CPAN" (
       http://apress.com/book/bookDisplay.html?bID=14 ) which is the best
       hands-on guide to creating module distributions.

       How do I create a class?

       See perltoot for an introduction to classes and objects, as well as
       perlobj and perlbot.

       How can I tell if a variable is tainted?

       You can use the tainted() function of the Scalar::Util module, avail-
       able from CPAN (or included with Perl since release 5.8.0).  See also
       "Laundering and Detecting Tainted Data" in perlsec.

       What's a closure?

       Closures are documented in perlref.

       Closure is a computer science term with a precise but hard-to-explain
       meaning. Closures are implemented in Perl as anonymous subroutines with
       lasting references to lexical variables outside their own scopes.
       These lexicals magically refer to the variables that were around when
       the subroutine was defined (deep binding).

       Closures make sense in any programming language where you can have the
       return value of a function be itself a function, as you can in Perl.
       Note that some languages provide anonymous functions but are not capa-
       ble of providing proper closures: the Python language, for example.
       For more information on closures, check out any textbook on functional
       programming.  Scheme is a language that not only supports but encour-
       ages closures.

       Here's a classic function-generating function:

	   sub add_function_generator {
	     return sub { shift() + shift() };
	   }

	   $add_sub = add_function_generator();
	   $sum = $add_sub->(4,5);		  # $sum is 9 now.

       The closure works as a function template with some customization slots
       left out to be filled later.  The anonymous subroutine returned by
       add_function_generator() isn't technically a closure because it refers
       to no lexicals outside its own scope.

       Contrast this with the following make_adder() function, in which the
       returned anonymous function contains a reference to a lexical variable
       outside the scope of that function itself.  Such a reference requires
       that Perl return a proper closure, thus locking in for all time the
       value that the lexical had when the function was created.

	   sub make_adder {
	       my $addpiece = shift;
	       return sub { shift() + $addpiece };
	   }

	   $f1 = make_adder(20);
	   $f2 = make_adder(555);

       Now "&$f1($n)" is always 20 plus whatever $n you pass in, whereas
       "&$f2($n)" is always 555 plus whatever $n you pass in.  The $addpiece
       in the closure sticks around.

       Closures are often used for less esoteric purposes.  For example, when
       you want to pass in a bit of code into a function:

	   my $line;
	   timeout( 30, sub { $line =  } );

       If the code to execute had been passed in as a string, '$line =
       ', there would have been no way for the hypothetical timeout()
       function to access the lexical variable $line back in its caller's
       scope.

       What is variable suicide and how can I prevent it?

       This problem was fixed in perl 5.004_05, so preventing it means upgrad-
       ing your version of perl. ;)

       Variable suicide is when you (temporarily or permanently) lose the
       value of a variable.  It is caused by scoping through my() and local()
       interacting with either closures or aliased foreach() iterator vari-
       ables and subroutine arguments.	It used to be easy to inadvertently
       lose a variable's value this way, but now it's much harder.  Take this
       code:

	   my $f = 'foo';
	   sub T {
	     while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" }
	   }
	   T;
	   print "Finally $f\n";

       If you are experiencing variable suicide, that "my $f" in the subrou-
       tine doesn't pick up a fresh copy of the $f whose value is . The
       output shows that inside the subroutine the value of $f leaks through
       when it shouldn't, as in this output:

	       foobar
	       foobarbar
	       foobarbarbar
	       Finally foo

       The $f that has "bar" added to it three times should be a new $f "my
       $f" should create a new lexical variable each time through the loop.
       The expected output is:

	       foobar
	       foobar
	       foobar
	       Finally foo

       How can I pass/return a {Function, FileHandle, Array, Hash, Method,
       Regex}?

       With the exception of regexes, you need to pass references to these
       objects.  See "Pass by Reference" in perlsub for this particular ques-
       tion, and perlref for information on references.

       See "Passing Regexes", below, for information on passing regular
       expressions.

       Passing Variables and Functions
	   Regular variables and functions are quite easy to pass: just pass
	   in a reference to an existing or anonymous variable or function:

	       func( \$some_scalar );

	       func( \@some_array  );
	       func( [ 1 .. 10 ]   );

	       func( \%some_hash   );
	       func( { this => 10, that => 20 }   );

	       func( \&some_func   );
	       func( sub { $_[0] ** $_[1] }   );

       Passing Filehandles
	   As of Perl 5.6, you can represent filehandles with scalar variables
	   which you treat as any other scalar.

		   open my $fh, $filename or die "Cannot open $filename! $!";
		   func( $fh );

		   sub func {
			   my $passed_fh = shift;

			   my $line = <$fh>;
			   }

	   Before Perl 5.6, you had to use the *FH or "\*FH" notations.  These
	   are "typeglobs"--see "Typeglobs and Filehandles" in perldata and
	   especially "Pass by Reference" in perlsub for more information.

       Passing Regexes
	   To pass regexes around, you'll need to be using a release of Perl
	   sufficiently recent as to support the "qr//" construct, pass around
	   strings and use an exception-trapping eval, or else be very, very
	   clever.

	   Here's an example of how to pass in a string to be regex compared
	   using "qr//":

	       sub compare($$) {
		   my ($val1, $regex) = @_;
		   my $retval = $val1 =~ /$regex/;
		   return $retval;
	       }
	       $match = compare("old McDonald", qr/d.*D/i);

	   Notice how "qr//" allows flags at the end.  That pattern was com-
	   piled at compile time, although it was executed later.  The nifty
	   "qr//" notation wasn't introduced until the 5.005 release.  Before
	   that, you had to approach this problem much less intuitively.  For
	   example, here it is again if you don't have "qr//":

	       sub compare($$) {
		   my ($val1, $regex) = @_;
		   my $retval = eval { $val1 =~ /$regex/ };
		   die if $@;
		   return $retval;
	       }

	       $match = compare("old McDonald", q/($?i)d.*D/);

	   Make sure you never say something like this:

	       return eval "\$val =~ /$regex/";   # WRONG

	   or someone can sneak shell escapes into the regex due to the double
	   interpolation of the eval and the double-quoted string.  For exam-
	   ple:

	       $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';

	       eval "\$string =~ /$pattern_of_evil/";

	   Those preferring to be very, very clever might see the O'Reilly
	   book, Mastering Regular Expressions, by Jeffrey Friedl.  Page 273's
	   Build_MatchMany_Function() is particularly interesting.  A complete
	   citation of this book is given in perlfaq2.

       Passing Methods
	   To pass an object method into a subroutine, you can do this:

	       call_a_lot(10, $some_obj, "methname")
	       sub call_a_lot {
		   my ($count, $widget, $trick) = @_;
		   for (my $i = 0; $i < $count; $i++) {
		       $widget->$trick();
		   }
	       }

	   Or, you can use a closure to bundle up the object, its method call,
	   and arguments:

	       my $whatnot =  sub { $some_obj->obfuscate(@args) };
	       func($whatnot);
	       sub func {
		   my $code = shift;
		   &$code();
	       }

	   You could also investigate the can() method in the UNIVERSAL class
	   (part of the standard perl distribution).

       How do I create a static variable?

       (contributed by brian d foy)

       Perl doesn't have "static" variables, which can only be accessed from
       the function in which they are declared. You can get the same effect
       with lexical variables, though.

       You can fake a static variable by using a lexical variable which goes
       out of scope. In this example, you define the subroutine "counter", and
       it uses the lexical variable $count. Since you wrap this in a BEGIN
       block, $count is defined at compile-time, but also goes out of scope at
       the end of the BEGIN block. The BEGIN block also ensures that the sub-
       routine and the value it uses is defined at compile-time so the subrou-
       tine is ready to use just like any other subroutine, and you can put
       this code in the same place as other subroutines in the program text
       (i.e. at the end of the code, typically). The subroutine "counter"
       still has a reference to the data, and is the only way you can access
       the value (and each time you do, you increment the value).  The data in
       chunk of memory defined by $count is private to "counter".

	   BEGIN {
	       my $count = 1;
	       sub counter { $count++ }
	   }

	   my $start = count();

	   .... # code that calls count();

	   my $end = count();

       In the previous example, you created a function-private variable
       because only one function remembered its reference. You could define
       multiple functions while the variable is in scope, and each function
       can share the "private" variable. It's not really "static" because you
       can access it outside the function while the lexical variable is in
       scope, and even create references to it. In this example, "incre-
       ment_count" and "return_count" share the variable. One function adds to
       the value and the other simply returns the value.  They can both access
       $count, and since it has gone out of scope, there is no other way to
       access it.

	   BEGIN {
	       my $count = 1;
	       sub increment_count { $count++ }
	       sub return_count    { $count }
	   }

       To declare a file-private variable, you still use a lexical variable.
       A file is also a scope, so a lexical variable defined in the file can-
       not be seen from any other file.

       See "Persistent Private Variables" in perlsub for more information.
       The discussion of closures in perlref may help you even though we did
       not use anonymous subroutines in this answer. See "Persistent Private
       Variables" in perlsub for details.

       What's the difference between dynamic and lexical (static) scoping?
       Between local() and my()?

       "local($x)" saves away the old value of the global variable $x and
       assigns a new value for the duration of the subroutine which is visible
       in other functions called from that subroutine.	This is done at
       run-time, so is called dynamic scoping.	local() always affects global
       variables, also called package variables or dynamic variables.

       "my($x)" creates a new variable that is only visible in the current
       subroutine.  This is done at compile-time, so it is called lexical or
       static scoping.	my() always affects private variables, also called
       lexical variables or (improperly) static(ly scoped) variables.

       For instance:

	   sub visible {
	       print "var has value $var\n";
	   }

	   sub dynamic {
	       local $var = 'local';   # new temporary value for the still-global
	       visible();	       #   variable called $var
	   }

	   sub lexical {
	       my $var = 'private';    # new private variable, $var
	       visible();	       # (invisible outside of sub scope)
	   }

	   $var = 'global';

	   visible();		       # prints global
	   dynamic();		       # prints local
	   lexical();		       # prints global

       Notice how at no point does the value "private" get printed.  That's
       because $var only has that value within the block of the lexical()
       function, and it is hidden from called subroutine.

       In summary, local() doesn't make what you think of as private, local
       variables.  It gives a global variable a temporary value.  my() is what
       you're looking for if you want private variables.

       See "Private Variables via my()" in perlsub and "Temporary Values via
       local()" in perlsub for excruciating details.

       How can I access a dynamic variable while a similarly named lexical is
       in scope?

       If you know your package, you can just mention it explicitly, as in
       $Some_Pack::var. Note that the notation $::var is not the dynamic $var
       in the current package, but rather the one in the "main" package, as
       though you had written $main::var.

	       use vars '$var';
	       local $var = "global";
	       my    $var = "lexical";

	       print "lexical is $var\n";
	       print "global  is $main::var\n";

       Alternatively you can use the compiler directive our() to bring a
       dynamic variable into the current lexical scope.

	       require 5.006; # our() did not exist before 5.6
	       use vars '$var';

	       local $var = "global";
	       my $var	  = "lexical";

	       print "lexical is $var\n";

	       {
		 our $var;
		 print "global	is $var\n";
	       }

       What's the difference between deep and shallow binding?

       In deep binding, lexical variables mentioned in anonymous subroutines
       are the same ones that were in scope when the subroutine was created.
       In shallow binding, they are whichever variables with the same names
       happen to be in scope when the subroutine is called.  Perl always uses
       deep binding of lexical variables (i.e., those created with my()).
       However, dynamic variables (aka global, local, or package variables)
       are effectively shallowly bound.  Consider this just one more reason
       not to use them.  See the answer to "What's a closure?".

       Why doesn't "my($foo) = ;" work right?

       "my()" and "local()" give list context to the right hand side of "=".
       The  read operation, like so many of Perl's functions and opera-
       tors, can tell which context it was called in and behaves appropri-
       ately.  In general, the scalar() function can help.  This function does
       nothing to the data itself (contrary to popular myth) but rather tells
       its argument to behave in whatever its scalar fashion is.  If that
       function doesn't have a defined scalar behavior, this of course doesn't
       help you (such as with sort()).

       To enforce scalar context in this particular case, however, you need
       merely omit the parentheses:

	   local($foo) = ;	   # WRONG
	   local($foo) = scalar();   # ok
	   local $foo  = ;	   # right

       You should probably be using lexical variables anyway, although the
       issue is the same here:

	   my($foo) = ;  # WRONG
	   my $foo  = ;  # right

       How do I redefine a builtin function, operator, or method?

       Why do you want to do that? :-)

       If you want to override a predefined function, such as open(), then
       you'll have to import the new definition from a different module.  See
       "Overriding Built-in Functions" in perlsub.  There's also an example in
       "Class::Template" in perltoot.

       If you want to overload a Perl operator, such as "+" or "**", then
       you'll want to use the "use overload" pragma, documented in overload.

       If you're talking about obscuring method calls in parent classes, see
       "Overridden Methods" in perltoot.

       What's the difference between calling a function as &foo and foo()?

       When you call a function as &foo, you allow that function access to
       your current @_ values, and you bypass prototypes.  The function
       doesn't get an empty @_--it gets yours!	While not strictly speaking a
       bug (it's documented that way in perlsub), it would be hard to consider
       this a feature in most cases.

       When you call your function as "&foo()", then you do get a new @_, but
       prototyping is still circumvented.

       Normally, you want to call a function using "foo()".  You may only omit
       the parentheses if the function is already known to the compiler
       because it already saw the definition ("use" but not "require"), or via
       a forward reference or "use subs" declaration.  Even in this case, you
       get a clean @_ without any of the old values leaking through where they
       don't belong.

       How do I create a switch or case statement?

       This is explained in more depth in the perlsyn.	Briefly, there's no
       official case statement, because of the variety of tests possible in
       Perl (numeric comparison, string comparison, glob comparison, regex
       matching, overloaded comparisons, ...).	Larry couldn't decide how best
       to do this, so he left it out, even though it's been on the wish list
       since perl1.

       Starting from Perl 5.8 to get switch and case one can use the Switch
       extension and say:

	       use Switch;

       after which one has switch and case.  It is not as fast as it could be
       because it's not really part of the language (it's done using source
       filters) but it is available, and it's very flexible.

       But if one wants to use pure Perl, the general answer is to write a
       construct like this:

	   for ($variable_to_test) {
	       if    (/pat1/)  { }     # do something
	       elsif (/pat2/)  { }     # do something else
	       elsif (/pat3/)  { }     # do something else
	       else	       { }     # default
	   }

       Here's a simple example of a switch based on pattern matching, this
       time lined up in a way to make it look more like a switch statement.
       We'll do a multiway conditional based on the type of reference stored
       in $whatchamacallit:

	   SWITCH: for (ref $whatchamacallit) {

	       /^$/	       && die "not a reference";

	       /SCALAR/        && do {
				       print_scalar($$ref);
				       last SWITCH;
			       };

	       /ARRAY/	       && do {
				       print_array(@$ref);
				       last SWITCH;
			       };

	       /HASH/	       && do {
				       print_hash(%$ref);
				       last SWITCH;
			       };

	       /CODE/	       && do {
				       warn "can't print function ref";
				       last SWITCH;
			       };

	       # DEFAULT

	       warn "User defined type skipped";

	   }

       See "perlsyn/"Basic BLOCKs and Switch Statements"" for many other exam-
       ples in this style.

       Sometimes you should change the positions of the constant and the vari-
       able.  For example, let's say you wanted to test which of many answers
       you were given, but in a case-insensitive way that also allows abbrevi-
       ations.	You can use the following technique if the strings all start
       with different characters or if you want to arrange the matches so that
       one takes precedence over another, as "SEND" has precedence over "STOP"
       here:

	   chomp($answer = <>);
	   if	 ("SEND"  =~ /^\Q$answer/i) { print "Action is send\n"	}
	   elsif ("STOP"  =~ /^\Q$answer/i) { print "Action is stop\n"	}
	   elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
	   elsif ("LIST"  =~ /^\Q$answer/i) { print "Action is list\n"	}
	   elsif ("EDIT"  =~ /^\Q$answer/i) { print "Action is edit\n"	}

       A totally different approach is to create a hash of function refer-
       ences.

	   my %commands = (
	       "happy" => \&joy,
	       "sad",  => \&sullen,
	       "done"  => sub { die "See ya!" },
	       "mad"   => \&angry,
	   );

	   print "How are you? ";
	   chomp($string = );
	   if ($commands{$string}) {
	       $commands{$string}->();
	   } else {
	       print "No such command: $string\n";
	   }

       How can I catch accesses to undefined variables, functions, or methods?

       The AUTOLOAD method, discussed in "Autoloading" in perlsub and
       "AUTOLOAD: Proxy Methods" in perltoot, lets you capture calls to unde-
       fined functions and methods.

       When it comes to undefined variables that would trigger a warning under
       "use warnings", you can promote the warning to an error.

	       use warnings FATAL => qw(uninitialized);

       Why can't a method included in this same file be found?

       Some possible reasons: your inheritance is getting confused, you've
       misspelled the method name, or the object is of the wrong type.	Check
       out perltoot for details about any of the above cases.  You may also
       use "print ref($object)" to find out the class $object was blessed
       into.

       Another possible reason for problems is because you've used the indi-
       rect object syntax (eg, "find Guru "Samy"") on a class name before Perl
       has seen that such a package exists.  It's wisest to make sure your
       packages are all defined before you start using them, which will be
       taken care of if you use the "use" statement instead of "require".  If
       not, make sure to use arrow notation (eg., "Guru->find("Samy")")
       instead.  Object notation is explained in perlobj.

       Make sure to read about creating modules in perlmod and the perils of
       indirect objects in "Method Invocation" in perlobj.

       How can I find out my current package?

       If you're just a random program, you can do this to find out what the
       currently compiled package is:

	   my $packname = __PACKAGE__;

       But, if you're a method and you want to print an error message that
       includes the kind of object you were called on (which is not necessar-
       ily the same as the one in which you were compiled):

	   sub amethod {
	       my $self  = shift;
	       my $class = ref($self) || $self;
	       warn "called me from a $class object";
	   }

       How can I comment out a large block of perl code?

       You can use embedded POD to discard it.	Enclose the blocks you want to
       comment out in POD markers.  The <=begin> directive marks a section for
       a specific formatter.  Use the "comment" format, which no formatter
       should claim to understand (by policy).	Mark the end of the block with
       <=end>.

	   # program is here

	   =begin comment

	   all of this stuff

	   here will be ignored
	   by everyone

	       =end comment

	   =cut

	   # program continues

       The pod directives cannot go just anywhere.  You must put a pod direc-
       tive where the parser is expecting a new statement, not just in the
       middle of an expression or some other arbitrary grammar production.

       See perlpod for more details.

       How do I clear a package?

       Use this code, provided by Mark-Jason Dominus:

	   sub scrub_package {
	       no strict 'refs';
	       my $pack = shift;
	       die "Shouldn't delete main package"
		   if $pack eq "" || $pack eq "main";
	       my $stash = *{$pack . '::'}{HASH};
	       my $name;
	       foreach $name (keys %$stash) {
		   my $fullname = $pack . '::' . $name;
		   # Get rid of everything with that name.
		   undef $$fullname;
		   undef @$fullname;
		   undef %$fullname;
		   undef &$fullname;
		   undef *$fullname;
	       }
	   }

       Or, if you're using a recent release of Perl, you can just use the Sym-
       bol::delete_package() function instead.

       How can I use a variable as a variable name?

       Beginners often think they want to have a variable contain the name of
       a variable.

	   $fred    = 23;
	   $varname = "fred";
	   ++$$varname; 	# $fred now 24

       This works sometimes, but it is a very bad idea for two reasons.

       The first reason is that this technique only works on global variables.
       That means that if $fred is a lexical variable created with my() in the
       above example, the code wouldn't work at all: you'd accidentally access
       the global and skip right over the private lexical altogether.  Global
       variables are bad because they can easily collide accidentally and in
       general make for non-scalable and confusing code.

       Symbolic references are forbidden under the "use strict" pragma.  They
       are not true references and consequently are not reference counted or
       garbage collected.

       The other reason why using a variable to hold the name of another vari-
       able is a bad idea is that the question often stems from a lack of
       understanding of Perl data structures, particularly hashes.  By using
       symbolic references, you are just using the package's symbol-table hash
       (like %main::) instead of a user-defined hash.  The solution is to use
       your own hash or a real reference instead.

	   $USER_VARS{"fred"} = 23;
	   $varname = "fred";
	   $USER_VARS{$varname}++;  # not $$varname++

       There we're using the %USER_VARS hash instead of symbolic references.
       Sometimes this comes up in reading strings from the user with variable
       references and wanting to expand them to the values of your perl pro-
       gram's variables.  This is also a bad idea because it conflates the
       program-addressable namespace and the user-addressable one.  Instead of
       reading a string and expanding it to the actual contents of your pro-
       gram's own variables:

	   $str = 'this has a $fred and $barney in it';
	   $str =~ s/(\$\w+)/$1/eeg;		 # need double eval

       it would be better to keep a hash around like %USER_VARS and have vari-
       able references actually refer to entries in that hash:

	   $str =~ s/\$(\w+)/$USER_VARS{$1}/g;	 # no /e here at all

       That's faster, cleaner, and safer than the previous approach.  Of
       course, you don't need to use a dollar sign.  You could use your own
       scheme to make it less confusing, like bracketed percent symbols, etc.

	   $str = 'this has a %fred% and %barney% in it';
	   $str =~ s/%(\w+)%/$USER_VARS{$1}/g;	 # no /e here at all

       Another reason that folks sometimes think they want a variable to con-
       tain the name of a variable is because they don't know how to build
       proper data structures using hashes.  For example, let's say they
       wanted two hashes in their program: %fred and %barney, and that they
       wanted to use another scalar variable to refer to those by name.

	   $name = "fred";
	   $$name{WIFE} = "wilma";     # set %fred

	   $name = "barney";
	   $$name{WIFE} = "betty";     # set %barney

       This is still a symbolic reference, and is still saddled with the prob-
       lems enumerated above.  It would be far better to write:

	   $folks{"fred"}{WIFE}   = "wilma";
	   $folks{"barney"}{WIFE} = "betty";

       And just use a multilevel hash to start with.

       The only times that you absolutely must use symbolic references are
       when you really must refer to the symbol table.	This may be because
       it's something that can't take a real reference to, such as a format
       name.  Doing so may also be important for method calls, since these
       always go through the symbol table for resolution.

       In those cases, you would turn off "strict 'refs'" temporarily so you
       can play around with the symbol table.  For example:

	   @colors = qw(red blue green yellow orange purple violet);
	   for my $name (@colors) {
	       no strict 'refs';  # renege for the block
	       *$name = sub { "@_" };
	   }

       All those functions (red(), blue(), green(), etc.) appear to be sepa-
       rate, but the real code in the closure actually was compiled only once.

       So, sometimes you might want to use symbolic references to directly
       manipulate the symbol table.  This doesn't matter for formats, handles,
       and subroutines, because they are always global--you can't use my() on
       them.  For scalars, arrays, and hashes, though--and usually for subrou-
       tines-- you probably only want to use hard references.

       What does "bad interpreter" mean?

       (contributed by brian d foy)

       The "bad interpreter" message comes from the shell, not perl.  The
       actual message may vary depending on your platform, shell, and locale
       settings.

       If you see "bad interpreter - no such file or directory", the first
       line in your perl script (the "shebang" line) does not contain the
       right path to perl (or any other program capable of running scripts).
       Sometimes this happens when you move the script from one machine to
       another and each machine has a different path to perl---/usr/bin/perl
       versus /usr/local/bin/perl for instance. It may also indicate that the
       source machine has CRLF line terminators and the destination machine
       has LF only: the shell tries to find /usr/bin/perl, but can't.

       If you see "bad interpreter: Permission denied", you need to make your
       script executable.

       In either case, you should still be able to run the scripts with perl
       explicitly:

	       % perl script.pl

       If you get a message like "perl: command not found", perl is not in
       your PATH, which might also mean that the location of perl is not where
       you expect it so you need to adjust your shebang line.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and other
       authors as noted. All rights reserved.

       This documentation is free; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in this file are
       hereby placed into the public domain.  You are permitted and encouraged
       to use this code in your own programs for fun or for profit as you see
       fit.  A simple comment in the code giving credit would be courteous but
       is not required.



perl v5.8.8			  2006-01-07			   PERLFAQ7(1)
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+112
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