I would rather sort the keys in ascending order (of key length), store them in an array, and compare the elements of that array, something like:

use strict;
use warnings;

my %h = (
     'this is a test' => 2,
     'is a test' => 2,
     'a test' => 1
    );

my @keys = sort {length $a <=> length $b} keys %h;

for ( 0 .. $#keys-1){
  if ($keys[$_+1] =~ /$keys[$_]/){
    delete $h{$keys[$_]};
  }
}
[download]

Hope this helps

citromatik