This module adds numeric error (or uncertainty) support to the 
Math::Symbolic parser. It does so by extending the parser grammar of the 
Math::Symbolic module (that is, the one stored in $Math::Symbolic::Parser) 
with certain special functions that create constants as Number::WithError 
objects. (Math::Symbolic::Variable objects have been able to contain 
objects since the very beginning.)

WWW: http://search.cpan.org/dist/Math-SymbolicX-Error/