$ prove -v graph.t
graph.t .. 
ok 1 - Constructor returned defined value
ok 2 - An object of class 'Graph' isa 'Graph'
ok 3 - vertex Jolt
ok 4 - vertex Lord
ok 5 - vertex Tornado
ok 6 - Add edge Tornado      -> Jolt
ok 7 - Add edge Tornado      -> Lord
not ok 8 - I expect that I cannot add Lord -> Tornado edge
#   Failed test 'I expect that I cannot add Lord -> Tornado edge'
#   at graph.t line 23.
1..8
# Looks like you failed 1 test of 8.
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/8 subtests 

Test Summary Report
-------------------
graph.t (Wstat: 256 Tests: 8 Failed: 1)
  Failed test:  8
  Non-zero exit status: 1
Files=1, Tests=8,  0 wallclock secs ( 0.01 usr  0.01 sys +  0.11 cusr  0.00 csys =  0.13 CPU)
Result: FAIL