Actually, I find that validating my parms is actually less useful in perl than it was in other languages. That's largely because perl is soooo much better at DWIMmery than other languages.
For example, validating that the object you just got is a file handle is a complete waste of time. The object may be a glob reference, may be an IO::Handle (or derived) object, or may be something else tie'd to work like a file handle. Don't worry about it - just use it for reading or writing (as appropriate), and things work out great. If a wrong parm is passed in, you'll see that quickly enough ;-).
For another example, validating the the object you just got is a scalar is another waste of time. What if it's an object that has all the proper overloads so I can do some really funky stuff with it? You'll never believe what someone who is more skilled than you can get out of your module - don't stop them.
OTOH, validating user input from, say, the web, is of paramount importance. You're letting some arbitrary user who may be malicious use your computing resources, so you should be careful that they can't misuse them. Depends on what you're validating. In your case, it looks like the former, so you may be pleasantly surprised when suddenly the code you've been using for weeks or months in a certain way suddenly works wonderfully in another way.
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