When in wondering about something that is (or seems to be) some kind of internal hacker or computer joke, always turn to The Jargon File. A quick lookup gives us: foo, bar and foobar.

Good explanations from a hacker perspective, lots of cross-references and examples of other metasyntactic variables. :)

Hope that gives something of an explanation. As for when used in perl programs, they are usually example, or temporary, mockup variables. :)

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You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.