sub swap {
  my ($a, $b) = \(@_);
  
  ( $$a, $$b ) = ( $$b, $$a );  
}
 
my ($x,$y)=(42,666);
 
$\="\n";
print "($x,$y)";    #> (42,666)
swap($x,$y);
print "($x,$y)";    #> (666,42)