Similar to Test::Warn there is also a module called Test::Exception
to test for calls to die.
Try calling fibonacci(); and writing a test for it.
examples/perl/t/fibonacci_dies.t
use strict;
use warnings;

use Test::More tests => 3;

use lib 'lib';
use MyTools;


{
    my $result = fibonacci(3);
    is($result, 2, 'fibonacci on 3');
}
{
    my $result = fibonacci();
    is($result, 1, 'fibonacci()');
}
{
    my $result = fibonacci(4);
    is($result, 3, 'fibonacci on 4');
}

examples/perl/t/fibonacci_dies.out
1..3
ok 1 - fibonacci on 3
Need to get a number
# Looks like you planned 3 tests but only ran 1.
# Looks like your test died just after 1.