Just a few comments:

• 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.