$ cat graph.t

#!/usr/bin/env perl
use strict;
use warnings;
use Graph;
use Test::More qw( no_plan );

my @vertices = qw( Jolt Lord Tornado );

my $g = Graph->new(vertices => [ @vertices ]);
ok(defined $g, "Constructor returned defined value");
isa_ok($g, 'Graph');
for my $v (@vertices) {
    ok($g->has_vertex($v), "vertex $v");
}

my @edges = (
    [ 'Tornado'     => 'Jolt' ],
    [ 'Tornado'     => 'Lord' ],
);
for my $e (@edges) {
    ok($g->add_edge(@{$e}), sprintf("Add edge %-12s -> %s", @{$e}));
}

##</code><code>##

$ 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
1..7
ok
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

##</code><code>##

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

##</code><code>##

$ 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