First of all, I had to change that first line to

while ($fc =~ /#(\d+)\s+(([^#\s]\S*\s+\S+\s+)*)/g ) {

to get it to work the way you claimed. Otherwise that first match just grabs everything up to r5.

I'm also still unclear on what sorts of things are expected on input, e.g., why is the split call using \n when the first regexp suggests that you're expecting things to be separated by general whitespace \s+ which may or may not have newlines in it (or have multiple newlines in it)?

E.g., it's possible that

join '', map {m/(\d*)\s+(?:\S+\s+\S+\s+)*(\S+\s+\S+\s+)$/ ? "#$1\n$2" : ()} split /^#/m, $fc

is what you want, but I can't tell for sure.