Only the basic sleep supports sleeping forever.

It makes no sense ask sleep-timer to return how much time till forever,
nor is there a need for sleep-till to calculate when the end of time is.

S29-functions.pod

@@ -255,9 +255,11 @@ an embedded interpreter, all memory must also be reclaimed.
 
 =item sleep
 
- multi sleep ( Real $for = Inf --> Nil )
+ multi sleep ( Real $for --> Nil )
+ multi sleep ( Whatever --> Nil )
 
-Attempt to sleep for up to C<$for> seconds. Implementations are obligated
+Attempt to sleep for up to C<$for> seconds, or forever when the argument is C<*>.
+Implementations are obligated
 to support sub-second resolutions if that is at all possible.  You may pass
 any of C<Int>, C<Num>, C<Rat>, or C<Duration> types as an argument, since those
 all do C<Real>, but regardless of which type you use, they are always scaled to
@@ -272,8 +274,7 @@ if you really just want to keep rolling over in bed until your alarm
 goes off at a particular time, use C<sleep-till> instead, since it
 is not subject to relative clock drift.
 
-All of these sleep functions work only on the current thread and also
-accept * (Whatever) as an indication to sleep indefinitely.
+All of these sleep functions work only on the current thread.
 
 =item sleep-timer