#!/usr/bin/perl

my %Deer = (
  Dasher  => 'Now',
  Dancer  => 'Now',
  Prancer => 'Now',
  Vixen   => 'and',
  Comet   => 'On',
  Cupid   => 'On',
  Donder  => 'On',
  Blitzen => 'and',
  Now => {
    Dasher  => 'Dancer',
    Dancer  => 'Prancer',
    Prancer => 'Vixen',
    'and'   => { Vixen => 'Comet' },
  },
  On => {
    Comet  => 'Cupid',
    Cupid  => 'Donder',
    Donder => 'Blitzen',
    'and'  => { Blitzen => 0 },
  },
);

local $away;
print dash('Dasher'), <DATA>;

sub dash {
  my($deer, $from) = @_;
  return unless $deer;
  local $str;
  my $next;
  my $harness = $Deer{$deer};
  unless ($harness eq 'and') {
    $next = $Deer{$Deer{$deer}}{$deer};
    $harness .= ',';
    $harness = "! $harness" if $away;
  }
  else {
    $next = $Deer{$from}{$harness}{$deer};
    $harness = " $harness";
  }
  $str .= "$harness $deer";
  $str .= "!\n" if $away;
  $away = !$away;
  $str . dash($next, $Deer{$deer});
}

__DATA__
To the top of the porch!
To the top of the wall!
Now dash away! dash away!
Dash away all!