Dorkish indeed. I don't see how that makes any sense.

First, it gives tons of warnings. Second, it doesn't make sense to treat an error signal as a valid value.

$ perl -wE'
   sub expensive_call { $_[0] == -1 ? undef : $_[0] }
   say $_ // "[undef]"
      for
         sort {
            ( $c{$a} //= expensive_call($a) )
               <=>
            ( $c{$b} //= expensive_call($b) )
         }
         0,1,-1,-1,0,2;
'
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
0
-1
-1
0
1
2
[download]

It would make more sense to handle the error, perhaps by dying or by using a sensible default.

$ perl -wE'
   sub expensive_call { $_[0] == -1 ? undef : $_[0] }
   say $_ // "[undef]"
      for
         sort {
            ( $c{$a} ||= expensive_call($a) // -1 )
               <=>
            ( $c{$b} ||= expensive_call($b) // -1  )
         }
         0,1,-1,-1,0,2;
'
-1
-1
0
0
1
2
[download]

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