Like the error message says, a limitation of (?<=re) and (?<!re) is that re must only match expressions of a known, fixed length. That means you can use ?, *, +, etc in re. Basically, the regexp engine needs to know how many characters to look back.

Some alternatives:

$d =~ s/(\d\d)(?=\d)/$1-/g;
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1 while $d =~ s/(?<=\d\d)(?=\d)/-/;
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$d = reverse($d);
$d =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
$d = reverse($d);
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Update: Added solutions.