The amazing Inline module offers help here, too. Not all <math.h> functions are available in Perl (even if you use POSIX;).

Here's how to print some erf() values, for instance (a function almost certainly available in your maths library):

perl -le 'use Inline C => q{ double erf(double); }, ENABLE => "AUTOWRA
+P"; print "$_ ",erf($_) for (0..5)'
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You can of course write it into your modules, and use it to access you own libraries.

Other C wrapping options are XS (of course) and Swig.