I looked at the MQSeries::Command page and it seems to me that it is targeted to MQSeries Administrators (since it allows you to inquire and create objects in the server). Regarding your question about qdepth, it is explained in the synopsis examples (CurrentQDepth is one of the elements of QAttrs) Quoting the POD:

 foreach my $qname ( @qnames ) {

      $attr = $command->InquireQueue
        (
         QName          => $qname,
         QAttrs         => [qw(
                               OpenInputCount
                               OpenOutputCount
                               CurrentQDepth  # < === here is what you need
                              )],
        ) or die "InquireQueue: " . MQReasonToText($command->Reason()) . "\n";

      print "QName = $qname\n";

      foreach my $key ( sort keys %$attr ) {
          print "\t$key => $attr->{$key}\n";
      }
  }