You forgot one more method of concatenation via substr.

1. substr($a, length($a), 0, $b);
or
2. substr($a, length($a)) = $b;

This concatenation is faster for very long strings.

For string size 10 - 100 symbols fastest way is $a . $b and join

For string size ~1 Mb fastest ways join, substr and $a.$b

For strings size ~1Gb :) fastest ways are substr, strange method "@{[$a, $b]}" :) and join

Join has good speed for all variants.