use warnings;
use strict;

my @words = kmers(3);

sub kmers
{
    my $k     = shift;
    my @bases = qw( A T G C );
    my @words = @bases;

    for my $i (1 .. $k - 1)
    {
        print "\nBEGIN \$i = $i:\n";
        print "  \@bases = @bases\n";
        print "  \@words = @words\n";

        my @newwords;

        for my $w (@words)
        {
            push @newwords, $w.$_ for @bases;
        }

        @words = @newwords;

        print "END   \$i = $i:\n";
        print "  \@bases = @bases\n";
        print "  \@words = @words\n";
    }

    return @words;
}