On AIX, Perl v5.8.2, the message is clearer:

Math::Complex::make: Cannot take real part of Math::BigFloat.

And as I expected, it goes away, producing 7.61401993629603e+24, when the use bignum is removed.

Math::Complex has no idea that you have invoked bignum and has incompatible methods for this type of data. Some obvious choices are:

1) override and rewrite all the required Complex methods to be bignum-aware (check if bignum is loaded in a new "new" method) or

2) settle for no bignum

Update: or rewrite the gamma function from scratch using bignum, for reference: http://mathworld.wolfram.com/GammaFunction.html