Then, Replace The Untestable Part

{
    my @snooze_args = ();
    my $snooze_called = 0;
    local *Power::Manager::snooze = sub {
        $snooze_called++;
        @snooze_args = @_;  # trap the arguments
        return 0;   # simulate successful system call
    };

    should_i_sleep($Min_Power - 1);
    is( $snooze_called, 1,  'snooze called once' );
    is( @snooze_args,   0,  '  called properly'  );
}

Check it was called

Check it got the right arguments

By changing the return value to non-zero we can simulate a failure

Very, very powerful technique

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