Math::Pari is a fine module, but suffers, I think, from the fact that PARI can't currently be compiled as a dll on windows, at least last time i tried. I would be happy to be corrected here.

Using Math::Pari will also presume a little familiarity with GP and GP scripts, which may not be something you wish to learn. I found GP fun, a little like mathematica without the graphics, but learning it properly, for ecc, will take a little time.

The absence of the native ability of perl to deal with large integer arithmetic has given rise to some of the most ingenious perl/<large integer capable software> hybrids I've ever seen - witness adam back's diminutive munitions page, 5 lines over which i spent 2 days. you could use bc instead of pari - but the new perl will include large integers as a basic type.

or, perhaps quickest, you could perlify the code in michael rosing's 'implementing elliptic curve cryptography', written in c, and use bigint as the basis for your large integer arithmetic. while not the fastest in terms of runtime, at least you'd have the advantage that you could debug it all in one spot/ide(if you use activestate).