Great kata! I've been slowly going through the test suite, now around 60 passing...

I noticed a very minor issue, not affecting the coding. In the visual representation of this test:

   CH2
    / \
 H2C   CH2
   \   /
   HC-C
   //  \
   O    COOH

2-oxocyclopentan-1-carboxylic acid = [('C', 6), ('H', 8), ('O', 3)]

The Hydrogen on the bottom of the cycle (on C-2) is on the wrong carbon. It should be on C-1:

   \   /
    C-CH
   //  \
   O    COOH

Perl tests seem too permissive, passing for a non rounded float result:

20; 0,0.23,0.46,0.69,0.92,1.15,1.38,1.61 -> shoud return 41, got 41.4
Test Passed

Calculating the area was the "easy" part, compared to covering all the invalid path scenarios, especially properly handling the mousehole.