B::Tree is a simplified version of B::Graph for demonstration.
perl -MO=Tree program | dot -Tps > tree.ps
This is a very cut-down version of B::Graph; it generates minimalist tree graphs of the op tree of a Perl program, merely connecting the op nodes and labelling each node with the type of op.
It was written as an example of how to write compiler modules for "Professional Perl", but I've found it extremely useful for creating simple op tree graphs for use in presentations on Perl internals.
It requires the CPAN GraphViz module and the GraphViz package from http://www.research.att.com/sw/tools/graphviz/. It takes no options.