sub func {
   print("f-pre:  $_[0], $_[1]\n");
   $_[0] = 5;
   $_[1] = 6;
   print("f-post: $_[0], $_[1]\n");
}

my $x = 3;
my $y = 4;
print("pre:    $x, $y\n");
func($x, $y);
print("post:   $x, $y\n");