Just to show another way that doesn't use regular expressions, you could use substr with a negative offset to work back from the end of the string using a loop statement modifier. Not as clear to the eye as BrowserUk's regex solution but hopefully of interest.

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