#!/usr/bin/perl -w
use strict;

my @AoH = (
       {a => 1,  b  => 2,  c  => 3},
       {a => 1,  b  => 2},
       {a => 1,  b  => 2,  c  => 3,  d  => 4},
       {a => 1}
);

my $count = 0;
print join("\t", $count++, keys(%{$AoH[$_]})), "\n" for
    sort { keys(%{$AoH[$b]}) <=> keys(%{$AoH[$a]}) } 0 .. $#AoH;