use strict;

sub array_from_component {
    my ( $component, $index ) = @_;

    return( "" ) if ( scalar( @{$component} ) <= $index );

    my @result = ();

    my @answer = array_from_component( $component, $index + 1 );

    if ( $component->[$index]->[1] ) {
        # go through each letter one at a time
        foreach my $char ( split( //, $component->[$index]->[0] ) ) {
            foreach ( @answer ) {
                push( @result, $char . $_ );
            }
        }
    } else {
        foreach ( @answer ) {
            push( @result, $component->[$index]->[0] . $_ );
        }
    }

    return( @result );
}

sub array_from_string( $ ) {
    my ( $str ) = @_;

    # split up string into components
    my @component = ();
    while ( $str =~ m/(\[)?([ATCG]+)(\])?/g ) {
        # $1 will be set if this was a bracketed string
        push( @component, [ $2, $1 ? 1 : 0 ] );
    }

    # component now holds all the components, 
    # now for some recursive fun
    my @answer = array_from_component( \@component, 0 );

    return( @answer );
}

# From
my $str2 = "ATC[TG]CC";

my @result = array_from_string( $str2 );
print( join( "\n", @result ) );
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