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;
}