* Looks like your signatures only lists about 100 viruses. Why not use
the signature database from OpenAntivirus like ClamAV does?

Because the File::Scan module uses perl regular expressions in signatures
to detect polymorphic virus or suspicious virus. For speed the position of 
signature in virus file is also used.

* Where is the signatures file stored for File::Scan?

The signatures are stored internally in Scan.pm file, but you can add new 
signatures to signatures.txt file in files directory and rebuild the 
module.

* Can anybody tell me how I can compile a stand alone virus scanner?

$ cd examples
$ perlcc -o virusscan scan.pl

(Perl v5.8.0 or greater)

or try the Perl Archive Toolkit (PAR)

$ cd examples
$ pp -o scanner scan.pl