alarm() with negative argument makes no sense.

Give by default a warning, do not set the alarm, and return undef.

(the signedness problem detected by Coverity, CID 104837)

alarm() takes and returns unsigned int, not signed.

In other words, the C library function alarm() cannot fail, ever.
See for example:

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

pod/perldiag.pod

@@ -7099,6 +7099,11 @@ You likely need to figure out how this multi-byte character got mixed up
 with your single-byte locale (or perhaps you thought you had a UTF-8
 locale, but Perl disagrees).
 
+=item %s() with negative argument
+
+(S misc) Certain operations make no sense with negative arguments.
+Warning is given and the operation is not done.
+
 =item Within []-length '%c' not allowed
 
 (F) The count in the (un)pack template may be replaced by C<[TEMPLATE]>

pp_sys.c

@@ -4708,13 +4708,30 @@ PP(pp_alarm)
 {
 #ifdef HAS_ALARM
     dSP; dTARGET;
-    int anum;
-    anum = POPi;
-    anum = alarm((unsigned int)anum);
-    if (anum < 0)
-	RETPUSHUNDEF;
-    PUSHi(anum);
-    RETURN;
+    /* alarm() takes an unsigned int number of seconds, and return the
+     * unsigned int number of seconds remaining in the previous alarm
+     * (alarms don't stack).  Therefore negative return values are not
+     * possible. */
+    int anum = POPi;
+    if (anum < 0) {
+        /* Note that while the C library function alarm() as such has
+         * no errors defined (or in other words, properly behaving client
+         * code shouldn't expect any), alarm() being obsoleted by
+         * setitimer() and often being implemented in terms of
+         * setitimer(), can fail. */
+        /* diag_listed_as: %s() with negative argument */
+        Perl_ck_warner_d(aTHX_ packWARN(WARN_MISC),
+                         "alarm() with negative argument");
+        SETERRNO(EINVAL, LIB_INVARG);
+        RETPUSHUNDEF;
+    }
+    else {
+        unsigned int retval = alarm(anum);
+        if ((int)retval < 0) /* Strictly speaking "cannot happen". */
+            RETPUSHUNDEF;
+        PUSHu(retval);
+        RETURN;
+    }
 #else
     DIE(aTHX_ PL_no_func, "alarm");
 #endif

t/lib/warnings/pp_sys

@@ -921,3 +921,12 @@ gmtime(NaN) too large at - line 6.
 gmtime(NaN) failed at - line 6.
 localtime(NaN) too large at - line 7.
 localtime(NaN) failed at - line 7.
+
+########
+# pp_sys.c [pp_alarm]
+alarm(-1);
+no warnings "misc";
+alarm(-1);
+
+EXPECT
+alarm() with negative argument at - line 2.