Man pages matching name math::symbolic::custom::defaultdumpers in section 3pm:

Math::Symbolic::Custom::DefaultDumpers.3pm.en	debian 10 / libmath-symbolic-perl
Math::Symbolic::Custom::DefaultDumpers.3pm.en	debian 11 / libmath-symbolic-perl
Math::Symbolic::Custom::DefaultDumpers.3pm.en	debian 12 / libmath-symbolic-perl