# eliminate duplicates in history and keep only the most recent
for(my $i=scalar(@history)-1;$i>=0;$i--) {
	$_ = $history[$i];
	!$history{$_} ? $history{$_}=1 : splice(@history,$i--,1);
  }

##</code><code>##

{
  my (%history, $histcount);   # Should I initialise $histcount = 0 or is that "Cargo cult"?

  sub addHist { $history{ $_[0] } = $histcount++; }
  sub getHist { return = grep{ $history{ $_ } == ($histcount - $_[0]); } keys %history; }
}

... # get $newvalue from somewhere
addHist( $newvalue );
... # deside we want the $n'th previous value
my $last_nth = getHist( $n );
...

##</code><code>##

for ( my $i=$#history; $i >=0; $i-- ) {  # for $i = (last index of @history) to 0
  $_ = $history[$i];                     # get the value from the position in @history
  !$history{$_}                          # test if that value has been seen before
    ? $history{$_} = 1                   # if it hasn't record it
    : splice @history, $i--, 1)          # if it has remove it from @history 
                                         # (reducing $i to compensate for its remove;
}

##</code><code>##

for my $i ($#history .. 0 ) {
  !exists $history{ history[$i] }        # use exists as testing its value against zero will 
                                         # autovivify (which is why we need to set its value to 1)
    ? $history{ $history[$i] }++         # autovivify and increment. (By using exists, the 
                                         # setting to 1 is redundant but...
    : splice @history, $i--, 1);         # the -- in this version would be bad!!  
                                         # Thou shalt not modify the iterator in a for loop!
}

##</code><code>##

for ($#history .. 0) {
    my $temp = pop @history;
    if (!exists $history{ $temp }) { 
        $history{ $temp }++;
        unshift @history;
    } # else nothing to do we've already removed it.
}

##</code><code>##

for ($#history .. 0) {
	my $temp = pop @history;
	next if exists $history{ $temp };
	$history{ $temp } ++;
	unshift @history;
}

##</code><code>##

for ( push @history, '~~SOME UNLIKELY VALUE~~', my $value= shift @history; 
	$value ne '~~SOME UNLIKELY VALUE~~';
	$value = shift @history ) 
{
	push @history, $value, $history{ $value }++ unless exists $history{ $value }
}

##</code><code>##

use constant SENTINEL => '~~SOME UNLIKELY VALUE~~';

for ( push @history, SENTINEL, my $value= shift @history; 
	$value ne SENTINEL;
	$value = shift @history ) 
{
	push @history, $value, $history{ $value }++ unless exists $history{ $value }
}

##</code><code>##

my %history;
....
# get a new value here and only add it if its not there.
$history{ $newvalue } = 1 unless exists $history{ $newvalue };
...

##</code><code>##

my (%history, $histcount);
....
# get a new value here and only add it if its not there.

$history{ $newvalue } = $histcount++ unless exists $history{ $newvalue };

##</code><code>##

$history{ $newvalue } = $histcount++;  
...

##</code><code>##

for my $i  (keys %history) {
	last if $history{ $i } == ($histcount - $n); 		
}
my $last_nth = $history{ $i }; 			

##</code><code>##

 # Can't use multiple modifiers. Basic Plus2 was nice!
my last_nth = do { $_ for (keys %history) } while ( $history{ $_ } == ($histcount - $n) );  

my $last_nth = $_ if $history{ $_ } == n for (keys %history); # Again $_ is local

##</code><code>##

my $last_nth;
for my $i  (keys %history) {
	last if ($last_nth = $history{ $i }) == ($histcount - $n); 
}

##</code><code>##

my last_nth = grep{ $history{ $_ } == ($histcount - $n } keys %history;

##</code><code>##

my ($last_nth) = grep{ $history{ $_ } == ($histcount - $n); } keys %history;

##</code><code>##

my (%history, $histcount);

# To add $newvalue
$history{ $newvalue } = $histcount++ unless exists $history{ $newvalue };	

# To retrieve the last-but-$n'th 
my ($last_nth) = grep{ $history{ $_ } == ($histcount - $n); } keys %history; 	

##</code><code>##

my (%history, $histcount); 	# Should I initialise $histcount = 0 or is that "Cargo cult"?

sub addHist { $history{ $_[0] } = $histcount++; }
sub getHist { return = grep{ $history{ $_ } == ($histcount - $_[0]); } keys %history; }

# then
... # get $newvalue from somewhere
addHist( $newvalue );

... # deside we want the $n'th previous value
my $last_nth = getHist( $n );
...

##</code><code>##

my (%history, $histcount); 	# Should I initialise $histcount = 0 or is that "Cargo cult"?

sub addHist { 
	my ($hashref, $histcount, $new ) = @_;  # Naming our parameters makes things clearer
	# validate
	$hashref->{ $new } = $histcount++; 
}

sub getHist { 
	my ($hashref, $histcount, $new ) = @_;
	#validate
	return = grep{ $hashref->{ $_ } == ($histcount - $n); } keys %history; 
}

##</code><code>##

... # get $newvalue from somewhere
addHist( \%history, $histcount, $newvalue );

... # deside we want the $n'th previous value
my $last_nth = getHist( \%history, $histcount, $n );
...

##</code><code>##

{
	my (%history, $histcount); 	# Should I initialise $histcount = 0 or is that "Cargo cult"?

	sub addHist { $history{ $_[0] } = $histcount++; }
	sub getHist { return = grep{ $history{ $_ } == ($histcount - $_[0]); } keys %history; }
}

##</code><code>##

... # get $newvalue from somewhere
addHist( $newvalue );

... # deside we want the $n'th previous value
my $last_nth = getHist( $n );
...