sleep() with negative argument makes no sense.

Give by default a warning, do not sleep, and return zero.

(the signedness problem detected by Coverity, CID 104844)

Other options would include not giving a warning at all,
giving a warning only if asked, and finally croaking.
(Python had this problem earlier, and chose croaking.)

Earlier discussion in
http://www.nntp.perl.org/group/perl.perl5.porters/2015/03/msg226304.html

See also:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/sleep.html
http://linux.die.net/man/3/sleep
https://www.freebsd.org/cgi/man.cgi?query=sleep&sektion=3

pp_sys.c

@@ -4749,7 +4749,16 @@ PP(pp_sleep)
 	PerlProc_pause();
     else {
 	duration = POPi;
-	PerlProc_sleep((unsigned int)duration);
+        if (duration < 0) {
+          /* diag_listed_as: %s() with negative argument */
+          Perl_ck_warner_d(aTHX_ packWARN(WARN_MISC),
+                           "sleep() with negative argument");
+          SETERRNO(EINVAL, LIB_INVARG);
+          XPUSHi(0);
+          RETURN;
+        } else {
+          PerlProc_sleep((unsigned int)duration);
+        }
     }
     (void)time(&when);
     XPUSHi(when - lasttime);

t/lib/warnings/pp_sys

@@ -930,3 +930,12 @@ alarm(-1);
 
 EXPECT
 alarm() with negative argument at - line 2.
+
+########
+# pp_sys.c [pp_sleep]
+sleep(-1);
+no warnings "misc";
+sleep(-1);
+
+EXPECT
+sleep() with negative argument at - line 2.