#$data = "five[5]=12345&ten[10]=12345=678=90&six[6]=123456";
 $data = "five[5]=12345&ten[12]=12345=678=90&six[6]=123456";

for (;;) {
   $data =~ s/(.+?)\[(\d+)\]=// or last;
   $hash{$1} = substr($data, 0, $2, '');
   $data =~ s/^&// or last;
}

die("Bad input\n") if length $data;

use Data::Dumper;
print Dumper(\%hash);