Not perfect, but how about:

x.pm

use strict;
use warnings;

sub killer { die "hasta la vista baby"; }
sub exiter { exit(42); }

1;
[download]

main

use warnings;
use strict;

use Test::More tests => 3;
use Test::Exception;
ok(require('x.pm'), 'loaded');
throws_ok( sub { killer () }, qr/hasta la vista baby/, q{Expected kill
+er to die});

BEGIN
{
    no strict 'refs';
    no warnings;
    *CORE::GLOBAL::exit = sub {
        die "exited with code ", $_[0] || 0;
    };
}

throws_ok( sub { exiter () }, qr/exited with code 42/, q{killer exited
+});
[download]

Fletch suggested that CORE::exit could be redefined, but I had no success with that.

Updated with the help from imp. Thanks (++)