Pure & impure functions

pure_and_impure.pl

# This is an impure function, calling double will return 2 the first time, then 4, then 8...
my $input = 1;

sub double {
  $input = $input * 2;
  
  return $input;
}

# This is a pure function, calling triple(1) will always return 3.
sub triple {
  my $n = shift;

  return $n * 3;
}