...
PREREQ_PM => {
  ...
  (`which graphviz`) ? (GraphViz => '2.08') : ())
},
...

[download]

Adapt as required to make it portable :-)

In reply to Re: Can ExtUtils::MakeMaker recommend to install optional modules? by DrHyde
in thread Can ExtUtils::MakeMaker recommend to install optional modules? by voj

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