my $one = 1;
my $two = 2;
my $three = 3;
sub direct {
  $_[0] += 10;
  $_[1] += 9;
}
sub indirect {
  my ($a,$b,$c) = @_;
  $a += 99;
  $b += 99;
}
print "\$one = $one\n"; # 1
direct($one, $two, $three);
print "\$one = $one\n"; # changed to 11. $_[0] was an alias to $one.
indirect($one, $two, $three);
print "\$one = $one\n"; # still 11. The $a was assigned a copy of the value. It's not an alias