my %hash;
$data = "five[5]=12345&ten[10]=12345=678=90&six[6]=123456"

my $reading_key = 1;
my $key;
my $expected_length;
my $value_length;
my $value;
while ( $reading_key
        ? $data =~ /([^=]*)(=)/g
        : $data =~ /([^=&]*)([&=]|$)/g ) {
    my $chunk = $1;
    my $separator = $2;

    if ( $reading_key ) {
        # I expect the chunk to be complete and to
        # contain a number in square brackets to indicate how many
        # non-&/= characters the value contains.
        ( $key, $expected_length )
            = ( $chunk =~ /^([^\[]+)\[(\d+)\]/ );
        $reading_key = 0;
        $value = '';
        $value_length = 0;
    }
    else {
        $value_length += length $chunk;
        $value .= $chunk . $separator;
        if ( $value_length >= $expected_length ) {
            $hash->{ $key } = $value;
            $reading_key = 1;
            $value
                = $value_length
                = $expected_length
                = undef;
        }
    }
}