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$ cat graph.t

...
for my $e (@edges) {
    ok($g->add_edge(@{$e}), sprintf("Add edge %-12s -> %s", @{$e}));
}

$ prove -v graph.t
graph.t .. 
...
All tests successful.
Files=1, Tests=7,  0 wallclock secs ( 0.02 usr  0.00 sys +  0.11 cusr 
+ 0.00 csys =  0.13 CPU)
Result: PASS

ok(! $g->add_edge( 'Lord' => 'Tornado '),
    "I expect that I cannot add Lord -> Tornado edge");

$ prove -v graph.t
graph.t .. 
...
  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