There are some applications where perl rocks, and other applications where I don't think it will do so well.

Unfortunately, workstation virus checking may just land in the 2nd category.

A virus checker has to blast through a LOT of data, very quickly, and apply complex checks to it.

Obviously, perl does "a LOT of data" and "complex checks" very well, and would probably be a great language for expressing detection rules in, but the speed would be a major issue.

Doing checks on an email stream might be more to perl's liking, though. Less data, and perl tools already exist to handle the stream...
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Mike