PDL (Perl Data Language), Pari (number theory) or Inline::Octave (matrix algebra) are good advanced math modules. Other good open source math software are R (statistics) and Maxima (symbolic math). A Perl/R interface is available at Omegahat.

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Update: Forgot to mention Math::MatrixReal. It's a pure Perl implementation of matrix algebra for real numbers (hence solves eigensystem for only symmetric case, not general case, which may take complex values). In addition, Math::LP if you ever need linear programming. Graph was also mentioned in a recent post "recursive complex structure or something like that?" (for visual display of "graph," go with GraphViz).