It sound like you just want to run a cvs command from cron nightly. You can just use a shell script:

#!/bin/sh
cd /home/you/cvs/stuff
/usr/bin/cvs update
/usr/bin/cvs diff
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You can do the same in perl if you want

#!/usr/bin/perl
my $CVS = '/usr/bin/cvs';
my $DIR = '/home/you/cvs/stuff';
chdir $DIR;
print `$CVS update`;
print `$CVS diff`;
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Make sure you run the job as you so you pick up your CVSROOT environment var OK. Backticks let you capture the command output, so with the print and cron will email you the output automagically.