my %allKeys;
foreach my $i ( 0 .. $#data ){
  my $hash = $data[$i];
  for (keys %$hash) {
    push @{$allKeys{$_}}, $i;
  }
}

foreach my $key ( sort keys %allKeys ){
  my $locations = $allKeys{$key};
  printf "Key '%s' was seen %d times at these indices: %s\n",
    $key, scalar @$locations, join(", ", @$locations);
}