{
  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 );
...