Aren't you assuming a specific tool was used to do the zipping, though? Or, is that a universal standard?

Try this (requires the Archive::Zip module):

use Archive::Zip;
$zip = Archive::Zip->new();
die "Invalid zip file!" if $zip->read("zipfile.zip") != AZ_OK;
[download]

Update:You may need some of the other Archive modules if you want to check for gzipped files.