sub fuzzy {
    my $string = shift;
    my @alt;
    for(my $i = 0; $i < length($string); $i++) {
        my $alt = $string;
        substr($alt, $i, 1) = 'N';
        push @alt, $alt;
    }
    return join '|', $string, @alt;   # original too
}

print fuzzy('GCGAT');