If EXPR is omitted, splits the $_ string.  If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace).  Anything matching PATTERN is taken to be a delimiter separating the fields.  (Note that the delimiter may be longer than one character.)

##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;

print "$_\n" for split ( "Hello World" );
 
print "$_\n" for split /\s+/, "Hello World";

##</code><code>##

Here are the places where Perl will assume $_ even if you don't use it:
__snip__
- The default iterator variable in a foreach loop if no other variable is supplied.

##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;

print "\n\n";
print "\$_ = $_\n";

print "\n\n";
for ( 1 .. 10 ) { print "\$_ = $_\n" }

print "\n\n";
print $_ for {};

print "\n\n";
print Dumper $_ for {};

print "\n\n";
my $count = 0;
for ( sort keys %{ $_ } ) { print $count++, ":$_{$_} $_\n" };