This (waitpid returning) may be the case with OP's legacy environment... Current perl appears to set up the signal handlers with SA_RESTART

Wow, anonymonk... I didn't know that. You're right (well, almost). It's not SA_RESTART, it's

PP(pp_waitpid)
{
    ...
    if (PL_signals & PERL_SIGNALS_UNSAFE_FLAG)
        result = wait4pid(pid, &argflags, optype);
    else {
        while ((result = wait4pid(pid, &argflags, optype)) == -1 && er
+rno == EINTR) {
      PERL_ASYNC_CHECK();
        }
    }
    ...
[download]

(pp.c)

There is some mention in perlipc: On systems that supported it, older versions of Perl used the SA_RESTART flag when installing %SIG handlers. This meant that restartable system calls would continue rather than returning when a signal arrived. In order to deliver deferred signals promptly, Perl 5.8.0 and later do not use SA_RESTART. Consequently, restartable system calls can fail (with $! set to "EINTR") in places where they previously would have succeeded. The default ":perlio" layer retries "read", "write" and "close" as described above; interrupted "wait" and "waitpid" calls will always be retried.

Well, I stand corrected, then.