Although not directly applicable to the problem at hand (which can be solved with a require) there is the if module (core in 5.8.0) which will use a Perl module if a condition holds. The only problem with this is that it happens at compile time, so couldn't be directly applied in your particular case.
HTH

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broquaint

update: doh, just noticed LTjake's node above. note to self - recaffeinate head before posting