Feeling a bit ashamed of having come up with a cruder and less flexible solution than Aristotle's fine example above, I dare show it anyway. Most notable drawback of this is its expectation of the ocurrence of only one single pair of brackets.

$str1 = q/ATC[TG]CC/;

$ans_of_str2 = all_ans( $str1 );

sub all_ans {
    my @results;
    my ( $pre, $multi, $post ) = split /\[(.*)\]/, $_[0];

    if ( $multi ) {
        my @chars = split //, $multi;
        for ( @chars ) {
            push @results, "${pre}${_}$post"
        }
        return [ @results ];
    }

    return [ $_[0] ];
}
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Cheers, Sören