Here's another approach -- still using substr(), but differently. 
sub gapify {
  my $s = shift;
  return
    join '-',
      map(substr($s, 0, $_, ''), @_),  # runs of the string
     $s;  # the remainder of the string
}

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japhy -- Perl and Regex Hacker

In reply to Re: Adding gaps to a sequence by japhy
in thread Adding gaps to a sequence by ariels

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