If you just want to know whether it exists you have little choice but to grovel over @INC.

If you try to eval "use Some::Module" then the module will be loaded and any side-effects it has (such as exporting) will happen.

I suppose if you were feeling particularly crazy you could try something like this ...

print 'the module exists' if(!system("perl -MSome::Module -e exit"));
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but if I saw code like that in production I'm afraid I'd have to shoot you.