Perhaps rovf's machine is one of the machines that doesn't?

He has Windows, and it works for Windows too.

>perl -MPOSIX=:sys_wait_h -le"my $pid = system 1, $^X=>(-e=>'sleep 5')
+; while (waitpid($pid, WNOHANG) == 0) { print 'waiting'; sleep 1; } p
+rint 'done'"
waiting
waiting
waiting
waiting
waiting
waiting
done
[download]

To avoid suspending your program I'm guessing the code should look something like this?

use POSIX qw( WNOHANG );

# Returns false if alive.
# Returns true if dead. (Reaps and sets $?)
# Throws expection on error. (Sets $!)
sub is_child_dead {
    my ($pid) = @_;
    my $rv = waitpid($pid, WNOHANG);
    die("waitpid: $!\n") if $rv == -1;
    return $rv;
}
[download]