The line uses a regular expression to capture the canonical name of the object under CN=ExternalContacts in the object name ($this_in) and saves it to $this_mail.
It works as follows:

($this_mail) # puts the value in a list context so that values capture
+d by the regex are saved in the list (only the first match will be sa
+ved as there is only one variable available) 
            = 
              $this_in =~ # apply a regular expression to $this_in
                          /CN=(.*),CN=ExternalContacts/ # capture the 
+bit in brackets
                                                     gio;  #apply glob
+ally (g), case insensitively (i) and only compile once (o)
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