Math::Symbolic::Custom::Transformation is an extension to the 
Math::Symbolic module. You're assumed to be remotely familiar with that 
module throughout the documentation.

This package implements transformations of Math::Symbolic trees using 
Math::Symbolic trees. I'll try to explain what this means in the following 
paragraphs.

Until now, in order to be able to inspect a Math::Symbolic tree, one had 
to use the low-level Math::Symbolic interface like comparing the top 
node's term type with a constant (such as T_OPERATOR) and then its 
operator type with more constants. This has changed with the release of 
Math::Symbolic::Custom::Pattern.

To modify the tree, you had to use equally low-level or even 
encapsulation-breaking methods. This is meant to be changed by this 
distribution.

WWW: http://search.cpan.org/dist/Math-Symbolic-Custom-Transformation/