sub pass_by_value {
  my $value = shift;
  print $value++, "\n";
}

sub pass_by_reference {
  my $ref = shift;
  print $$value++, "\n";
}

my $x = 0;
pass_by_value($x) for ( 1 .. 5 );
pass_by_reference(\$x) for ( 1 .. 5 );