Here's a more memory efficient solution that only works if the keys are already sorted:

{
   my $last;
   my @list;
   local $, = ',';
   local $\ = $/;
   while (<>) {
      chomp;
      my ($key, $val) = split($,, $_, 2);
      print $last, splice(@list) if ($.!=1 && $key ne $last);
      $last = $key;
      push(@list, $val);
   }
   print $last, @list if (@list);
}
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I like the symetry of $/ and $, being used for both input and output.