No external loop, no reverse and nothing but "-" in the substitution part: 

$d =~ s/(?!^|$)(?(?{ pos() % 2 })(?!))/-/g;

[download]

Update: Bug fix. I originally forgot to prevent "-" from being placed at the end.

In reply to Re^2: Another puzzled RegEx Problem. by ikegami
in thread Another puzzled RegEx Problem. by pysome

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