Your program:

use strict;
use warnings;
use Benchmark;
my @junk = `cat /etc/motd`; # not available on windows
my $count = 10_000;
timethese($count, {
   'map' => sub { 
                 my @a = @junk;
                 map { s/a/b/ } @a;
                 return @a
                },
   'for' => sub {
                 my @a = @junk;
                 for (@a) { s/a/b/ };
                 return @a
                },
    }
);

# "my $var = defined(<STDIN>)";
[download]

wouldn't work because there's no cat and no /etc/motd available on your box (probably)

Other reasons: there were loads of typos (corrected).

Regards

mwa