Presumably because $@ is no longer true or perhaps $session never gets defined without actually throwing the exception you are trying to catch.... If you do this you save the double test on $@ and it should work just fine.

$session = '';
eval { $session = PHP::Session->new($cookies{PHPSESSID}->value); };
if ( ! $session or $@ ) {
    print $query->redirect(-location=>"$redirecturl"."0");
  exit 0;
}

$id = $session->id;
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