At the time of this review, 98% of CPAN testers report non-success. It's a shame there is no alternative interface to Graphviz on CPAN. Instead of interfacing to Graphviz via the "dot" command-line program, this really deserves an XS binding to the C libraries.
The function actually contains no real logic, as the author has instead hosted the logic as a webservice of his control (and it's not open source). It makes a web request for each call, so it useless for all but trivial cases, though it does at least cache responses. This should either be documented or it should be renamed into the WebService namespace so people are not misled.
Also, there is already a preexisting module called 'underscore' and since case-sensitive filesystems are not unpopular, that should be taken into account when naming a module. An OSX user, for example, cannot install both underscore and Underscore.
This shouldn't have been placed under a top-level namespace. The author already was using WWW::Reddit. If changing namespace for pedantic reasons, it should have been placed under the WebService namespace.