use warnings; use strict;

 sub kmers {
     my $k = shift;
     my $alphabet = qw( A T G C );
     my @bases = @$alphabet;
     my @words = @bases;
     for ( 1 .. --$k ) {
         my @newwords;
         foreach my $w (@words) {
             foreach my $b (@bases) {
                 push (@newwords, $w.$b);
             }
         }
         @words = @newwords;
     }
     return @words;
 }