Man pages matching name math::gsl::matrixcomplex in section 3pm:

Math::GSL::MatrixComplex.3pm.en	debian 10 / libmath-gsl-perl
Math::GSL::MatrixComplex.3pm.en	debian 11 / libmath-gsl-perl
Math::GSL::MatrixComplex.3pm.en	debian 12 / libmath-gsl-perl