The question doesn't have an answer, because your initial hash cannot exist. Hash keys have to be unique, otherwise they overwrite each other. So after your initial assignment, %pets looks like this:

%pets = (
          'pony' => 2,
          'cat' => 2,
          'dog' => 2
        );
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Now, if you had the original list assigned into an array, that would be a different story:

#!/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";
}
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Hope that helps!