This might be a mistake by the author, but
$] is treated as special variable here. Specifically, it is an alias of the variable
$OLD_PERL_VERSION which contains the version of perl encoded as number, e.g.
5.020002 in my system (corresponding to version number 5.20.2). So that value is inserted in the regex and it is matched literally. So for example, in my system, the above regex will only successfully match against strings that contain the substring