knoppix@Microknoppix:~$ perl -E '
> $str = q{abcdefghi};
> substr $str, -( $_ + 3 * ( $_ + 1 ) ), 0, ( q{-}, q{ } )[ $_ ]
>    for 0 .. 1;
> say $str;'
abc def-ghi
knoppix@Microknoppix:~$