Something like this perhaps:

#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
  chomp;

  my @a = split;

  my $same = 1;
  for (1 .. $#a) {
    if ($a[$_] ne $a[$_ - 1]) {
      $same = 0;
      last;
    }
  }

  print "@a - elements are ", $same ? "the same\n" : "different\n";
}

__DATA__
A A A A
A B C
A A B C
[download]

Or, if you like something a bit more esoteric:

#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
  chomp;

  my @a = split;

  my %h;
  @h{@a} = @a;
  my $same = keys %h == 1;

  print "@a - elements are ", $same ? "the same\n" : "different\n";
}

__DATA__
A A A A
A B C
A A B C
[download]