whats is this line doing with the input above

            (($this_mail) = ($this_in =~ /CN=(.*),CN=ExternalContacts/
+gio));
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Why don't you check yourself? Just add a

print ">>>$this_mail<<<\n";
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directly after your line. Then you'll see. The >>><<< are just visual delimiters - they help you to spot if your variable has some leading or trailing blanks.

If you have trouble understanding regexes, try this web-resource.

Or read some of the docs in this collection of planetscape

HTH, Rata