Your first while loop already runs through the whole file while not doing anything useful. Therefore the second while loop has nothing to do. Remove the first while loop altogether.
The statement while (<FILE>) of your second loop (now the only one...) will read a line at a time and assign it to $_. So you need to work with $_ within the loop block.
Your line $start =~ m/:(\d+)/; is applying the regex to the variable $start but you need to apply it to $_. Sou you might say $_ =~ m/:(\d+)/; which would work. It would be more Perlish to just say /:(\d+)/; as this would be applied to $_ by default.
The result of this match is that what was found in (...) is assigned to $1 so you need to write that to your file: print OFILE "$1\n";.
This is probably not all but if you re-read the earlier thread you should find more best practices.