The default input and pattern-searching space. The following pairs are equivalent:
while (<>) {...} # equivalent only in while!while (defined($_ = <>)) {...}/^Subject:/$_ =~ /^Subject:/tr/a-z/A-Z/$_ =~ tr/a-z/A-Z/chompchomp($_)Here are the places where Perl will assume$_ even if you don't use it:
The following functions use$_ as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.
All file tests (-f,-d) except for-t, which defaults to STDIN. See"-X" in perlfunc
The pattern matching operationsm//,s/// andtr/// (akay///) when used without an=~ operator.
The default iterator variable in aforeach loop if no other variable is supplied.
The implicit iterator variable in thegrep() andmap() functions.
The implicit variable ofgiven().
The default place to put the next value or input record when a<FH>,readline,readdir oreach operation's result is tested by itself as the sole criterion of awhile test. Outside awhile test, this will not happen.
$_ is a global variable.
However, between perl v5.10.0 and v5.24.0, it could be used lexically by writingmy $_. Making$_ refer to the global$_ in the same scope was then possible withour $_. This experimental feature was removed and is now a fatal error, but you may encounter it in older code.
Mnemonic: underline is understood in certain operations.
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