In which case, one might consider a local $SIG{__WARN__} handler:

sub is_zip {
   #Load the file
   local $SIG{__WARN__}  = sub { goto CONTINUE };
   $status = $zip->read($file);
   CONTINUE:
   #Check $status
}
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The code above is bad (it's an idea only), but the basic idea is to have a __WARN__ handler that supresses printing of the error and then branches to a goto tag outside itself, which keeps the default handler from being called.

However, using mime-magic to determine the file type is probably better.

#Read data into $file
if ($file =~ m/^\x50\x4B\x03\x04/) {
   #Parse as ZIP file
} else {
   #Parse as something else
}
[download]

Or automate that type of task with the previously-mentioned File::Type module.