#!/usr/bin/perl -w

use strict;

my @pets = ("dog", 1, "cat", 2, "pony", 2, "dog", 2);
my %pets; 

while (@pets) {
  my $key = shift @pets;
  my $val = shift @pets;

  $pets{$key} += $val;
}

foreach (sort keys %pets) {
  print "$_: $pets{$_}\n";
}