gistfile1.txt

#!/usr/bin/perl

use strict;
use warnings;

use feature 'say';

# Notice that the values are all equal for some of the keys
my @stuff = (
  { a => "1", b => "2", c => '4'},
  { a => "1", b => "3", c => '4'},
  { a => "1", b => "2", c => '4'},
  { a => "1", b => "2", c => '4'},
  { a => "1", b => "2", c => '4', d => '2'},
);

my %hash_count;

foreach my $ref_row (@stuff) {
  foreach my $key_letter (keys %$ref_row) {
    $hash_count{$key_letter}{ $ref_row->{$key_letter} }++;
  }
}

my @keys_total = keys %hash_count;
say "All the keys in \@stuff: @keys_total";

my @keys_equal;
foreach my $key_letter (keys %hash_count) {
  if (keys %{ $hash_count{$key_letter} } == 1) {
    push @keys_equal, $key_letter;
  }
}

say "The keys for which all values are equal: @keys_equal";

This looks

std::cout << "very good!" << std::endl;

Indeed.