The problem is that the regex consumes the characters it matches (so the next match happens after those characters). Use a look-aahead to not consume:

while ($str =~ /(?=(\w{3}bc\w{2}))/g) {
  print "$-[1],$+[1],$1\n";
}
[download]

Update: extra ')' added to regex (thanks SamCG and prasadbabu!)