Well, duh. Of course it gives a ton of warnings. But that's not the fault of the orkish maneuver - it's the fault of the set of elements you want to sort. You'd get the same warnings if you'd used a plain sort, or an ST - the warnings come from returning undefined values in the function that maps elements to sort to <=> operands.

#!/usr/bin/perl

use 5.010;

use strict;
use warnings;

sub expensive_call { $_[0] == -1 ? undef : $_[0] }
say $_ // "[undef]" for sort {expensive_call($a) <=> expensive_call($b
+)}
                             0,1,-1,-1,0,2;
__END__
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
0
-1
-1
0
1
2
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I complete fail to get the point you're trying to make.