#!/usr/bin/perl -w

use strict;

my @list = qw{ key value marker key };
my %seen = ();
my @newlist = ();

foreach my $item (@list) {
  if (!$seen{$item}) {
    $seen{$item} = 1;
    push(@newlist, $item);
  } else {
    push(@newlist, "$item" . "$seen{$item}");
    $seen{$item}++;
  }
}

print @newlist;