Lookbehind has to be a fixed width; yours isn't (the + quantifier) so it's an invalid regex.

This is a little kludgy, but will work:

my $str = "123456789";
$str = reverse $str;
$str =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
$str = reverse $str;
print $str, "\n";
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