My approach would be to fork and check the exit. You'd probably want to add a timeout feature to kill tenacious subs that fail to exit. Really stripped-down example:

use strict;
use warnings;

sub test_exit {
  my ($subr, $exit_val) = @_;
  my $subp = fork;
  if ($subp) {
    my $done = waitpid($subp, 0);
    print "Process $done Exited with ".($?>>8).", expected $exit_val\n
+";
  }
  else {
    $subr->();
    warn "Did not exit\n";
    exit (~$exit_val);
  }
}

test_exit(sub{print "Process has run\n"; exit(42)}, 42);
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