I am attempting to use the Graph module from CPAN for the first time. I am having trouble understanding how the module implements the concept of a "directed graph".

Suppose I have 3 nodes, "Jolt", "Lord", and "Tornado". I want to indicate that "Tornado" depends on each of "Jolt" and "Lord". Based on the Graph distribution's t/06_new.t and t/18_add_edge.t, I believe I can say that with the following program:

$ 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}));
}
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When I run this, I get this output:

$ 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
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So far so good. But to me "depends on" means that I should not be able to add an edge from "Jolt" to "Tornado" or from "Lord" to "Tornado" because I've already established the other direction for those edges. So my expectation is that, I when I add the following unit test:

ok(! $g->add_edge( 'Lord' => 'Tornado '),
    "I expect that I cannot add Lord -> Tornado edge");
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... the test will PASS because it is phrased in the negative. Instead, the test FAILs:

$ 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
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So, what am I not grokking about Graph and directed graphs? Is it possible to implement what I want with Graph, or is some other module better for this?

Thank you very much.