use strict;
use warnings;

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

my @AoH_sorted = sort { (keys(%{$b})) <=> (keys(%{$a})) } @AoH;

foreach my $i (0 .. $#AoH_sorted) {
  print "$i\t";
#  if (exists  $AoH_sorted[$i]) { # no need to check if exist when using foreach
    my %hash = %{$AoH_sorted[$i]};
    foreach my $key (keys %hash) {
      print "$key\t";
    }
#  }
  print "\n";
}