use strict;
use warnings;

use feature qw{ say };

say q{-} x 9;
say for @{ expand( q{AGCT} ) };
say q{-} x 9;
say for @{ expand( q{AXCT} ) };
say q{-} x 9;
say for @{ expand( q{CTACAGGAA} ) };
say q{-} x 9;

sub expand
{
    my $seq = shift;
    
    my %translate = (
       A => [ qw{ G C T } ],
       G => [ qw{ A C T } ],
       C => [ qw{ A G T } ],
       T => [ qw{ A G C } ],
       );

    my $raExpands = [];
    for my $posn ( 0 .. length( $seq ) - 1 )
    {
        my $letter = substr $seq, $posn, 1;
        next unless $letter =~ m{[AGCT]};

        my $copy = $seq;
        push
           @{ $raExpands },
           map {
               substr $copy, $posn, 1, $_;
               $copy;
               } @{ $translate{ $letter } };
    }

    return $raExpands;
}

##</code><code>##

---------
GGCT
CGCT
TGCT
AACT
ACCT
ATCT
AGAT
AGGT
AGTT
AGCA
AGCG
AGCC
---------
GXCT
CXCT
TXCT
AXAT
AXGT
AXTT
AXCA
AXCG
AXCC
---------
ATACAGGAA
GTACAGGAA
TTACAGGAA
CAACAGGAA
CGACAGGAA
CCACAGGAA
CTGCAGGAA
CTCCAGGAA
CTTCAGGAA
CTAAAGGAA
CTAGAGGAA
CTATAGGAA
CTACGGGAA
CTACCGGAA
CTACTGGAA
CTACAAGAA
CTACACGAA
CTACATGAA
CTACAGAAA
CTACAGCAA
CTACAGTAA
CTACAGGGA
CTACAGGCA
CTACAGGTA
CTACAGGAG
CTACAGGAC
CTACAGGAT
---------