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";
[download]

Hmmm, that missed the AAA's on the output (thanks Roy Johnson)

Here's another one just for fun :)

my $string = "ATCGC[ATCG]AAA[GA]";
my @out = "";
$string=~s!([^\[]*)(\[([^\[]*)\])?!@out=map{my$c=$_;map{$_.$1.$c}@out}
+split'',$3if$1or$3!ge;

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