How to list differences between Perforce labels

#!/bin/bash

NOW="/tmp/now_tmp"
BEFORE="/tmp/before_tmp"
LABEL=$1

USAGE="usage: "${0%}" [label]"
if [ "x$LABEL" == "x" ]; then echo $USAGE; exit; fi

p4 changes -l -i ./... > $NOW
p4 changes -l -i ./...@$LABEL &gt $BEFORE
for change_num in $(diff $NOW $BEFORE | perl -lne'/. Change (\d+)/ && print $1');
do
    p4 describe $change_num |perl -pne'do { print "-----------------------------------------------------------\n\n"; exit } if m{Differences \.\.\.}'
done