while (<FOO_FILE>){
       if  (/$foo/){
            $that = 1;
       }
       if  (/$bar/){
            last;  # emergency exit from the loop ;-)
       }
       if  ($that){
             print $_;
       }
  }

##</code><code>##

  while (<FOO_FILE>){ 
       last if /$foo/;
       # ignore all lines until we find foo
  }

  while (<FOO_FILE>){
       last if  /$bar/;
       print $_;
       # do something with the lines we find now
  }

  close FOO_FILE;
  # just ignore the rest of the file