sub compare {
   my $x = shift;
   print "\$a = $a, \$b = $b, \$x = $x, \@_ = @_\n";
   $a cmp $b;
}
sub test { sort compare 'z','a','b' }
my @z = test('foo', 'bar', 'baz');
print "\@z = @z\n";

output:
$a = z, $b = a, $x = foo, @_ = bar baz
$a = a, $b = b, $x = bar, @_ = baz
$a = b, $b = z, $x = baz, @_ = 
@z = a b z