#!/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