A note for anyone following along at home.

The poster has left out the fact that the script has

  use English;
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At the top. See perlvar for more info.

Probably most perl programmers write the above snippet like this (with error checking too):-

$( = $) = $new_gid;
die "Failed to changed groupid to $new_gid: $!"
    if $) != $new_gid || $( != $new_gid;
$< = $> = $new_uid;
die "Failed to changed userid to $new_uid: $!"
    if $> != $new_uid || $< != $new_uid;
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At least that is how I would write it not being a big fan of use English!