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

sub swap {
  my $a = \shift;
...
  ( $$a, $$b ) = ( $$b, $$a );  
}

sub swap { 
   ($_[0], $_[1] ) = ( $_[1], $_[0] ); 
}