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!
}