#! /usr/bin/perl -w

$digits = 4;                                                  
$order  = 4213;
$arrange= 9876;
$i      = 0;

while($i < $digits){
  $hash{(split(//,$order))[$i]} = (split(//,$arrange))[$i];
  $i++;
}

foreach (sort(split(//,$order))){
  printf("%d  =>  %d\n",$_,$hash{$_});
}