=over

=item $ARG

=item $_
X<$_> X<$ARG>

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/

    chomp
    chomp($_)

Here are the places where Perl will assume $_ even if you
don't use it:

=over 3

=item *

Various unary functions, including functions like ord() and int(), as well
as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
STDIN.

=item *

Various list functions like print() and unlink().

=item *

The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
without an C<=~> operator.

=item *

The default iterator variable in a C<foreach> loop if no other
variable is supplied.

=item *

The implicit iterator variable in the grep() and map() functions.

=item *

The default place to put an input record when a C<< <FH> >>
operation's result is tested by itself as the sole criterion of a C<while>
test.  Outside a C<while> test, this will not happen.

=back

As C<$_> is a global variable, this may lead in some cases to unwanted
side-effects.  As of perl 5.9.1, you can now use a lexical version of
C<$_> by declaring it in a file or in a block with C<my>.  Moreover,
declaring C<our $_> restores the global C<$_> in the current scope.

(Mnemonic: underline is understood in certain operations.)

=back