"Do you mind explaining the following piece of code please ?"

Well, you've listed two pieces of code which appear in different parts of the script I posted:

"'uid=(' . join('|', @users) . '),';"

I've already provided a link to what's going on here: "... create a regex with a capture group containing an alternation of all the users. If that's unfamiliar to you, see "Perl regular expressions tutorial".". Was there something in that documentation that you didn't understand?

See "Perl functions A-Z" for information about join (or any of the other core functions I've used).

You could always run the code I provided with a print statement to see how that expression is evaluated.

"$user = /$user_re/ ? $1 : ''"

That's the ternary (aka conditional) operator: see "perlop: Conditional Operator".