my $string = "ATCGC[ATCG]AAA[GA]";

my @out = ($string =~ m/([^\[]*)/)[0];
while ($string =~ m/\[(\w+)\]/g) {
	@out = map {my $char=$_;map {$_.$char} @out} split//,$1;
}

print join("\n", @out) . "\n";