Maybe when replying to somebody who already says is new to perl, it would be nice to actually test your code.

Problems:

1) don't pass $a and $b to a sort sub, since they are special variables

2) don't treat a hash like a hashref, it just won't work

3) you don't need the return either

use strict;
use warnings;

my %month_sort =
  ( 'JAN' =>  1, 'FEB' =>  2, 'MAR' =>  3,
    'APR' =>  4, 'MAY' =>  5, 'JUN' =>  6,
    'JUL' =>  7, 'AUG' =>  8, 'SEP' =>  9,
    'OCT' => 10, 'NOV' => 11, 'DEC' => 12
   );

sub sort_cmp {
      ( substr($a, 3, 2) cmp substr($b, 3, 2) )
      ||
      ( $month_sort{substr($a, 0, 3)} <=>
        $month_sort{substr($b, 0, 3)}
      );
}

my (@months) = qw(APR12 MAR13 APR11 MAR12 FEB13 APR13);

@months = ( sort sort_cmp @months);
print "@months \n";
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