This seems too simple, which makes me suspicious that I've missed something?

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

$hash{ $1 } = $2 while $data =~ m/ 
    ( [a-z]+ ) 
    \[ \d+ \] = 
    (.*?) 
    (?= (?: & [a-z]+ \[ ) | $ )
/gx;

print Dumper \%hash;;
$VAR1 = {
          'six' => '123456',
          'five' => '12345',
          'ten' => '12345=678=90'
        };
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