use strict;
use warnings;

use 5.014;

my $raItems = [ qw{ A T G C } ];
my $raTerms = [];

makeCombos( $raTerms, 3, $raItems );
say for @$raTerms;

sub makeCombos
{
    my( $raTerms, $iters, $raItems, @terms ) = @_;

    my @localTerms;
    if ( @terms )
    {
        push @localTerms,
           map {
               my $item = $_;
               map {
                   $_ . $item
                   } @terms
               } @$raItems;
    }
    else
    {
        push @localTerms, @$raItems;
    }

    $iters --;
    push @$raTerms, @localTerms;

    return
       $iters
       ? makeCombos( $raTerms, $iters, $raItems, @localTerms )
       : ();
}