#! perl -slw
use strict;

sub p{
    my @a = split '', shift;
    while( @_ ) {
        my @c= split '', shift;
        my @b=();
        for my $c ( @c ) {
            push @b, $_ . $c for @a ;
        }
        @a = @b;
    }
    return @a;
}

my @combs = (
    p( 'NT', 'GCATN', 'NG', 'NC', 'NG', 'NT', 'NG' ),
    p( 'NG', 'NT', 'NG', 'NC', 'NG', 'GCATN', 'NT' ),
    p( 'NA', 'GCATN', 'NC', 'NG', 'NC', 'NA', 'NC' ),
    p( 'NC', 'NA', 'NC', 'NG', 'NC', 'GCATN', 'NA' ),
);

my %seen; @seen{ @combs } = ();

print "The number of unique 7-char sequences that can be matched by the regexes is:",
    scalar keys %seen;