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

#!/usr/bin/perl
use warnings;
...
print "$_\n" for split ( "Hello World" );
 
print "$_\n" for split /\s+/, "Hello World";

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.

#!/usr/bin/perl
use warnings;
...
my $count = 0;
for ( sort keys %{ $_ } ) { print $count++, ":$_{$_} $_\n" };