use Fcntl qw(:DEFAULT :flock);
if (open(FILE, "+< stuff.txt"))
#if (sysopen(FILE, "stuff.txt", O_RDWR))
{
#    if (flock(FILE, LOCK_EX|LOCK_NB))
    {
        local $/ = undef;
        my $buff = <FILE>;
#        my $buff = '';
#        sysread(FILE, $buff, -s FILE);

        # DO STUFF TO $buff HERE, can GROW or SHRINK
        # Split it into pieces...

        seek(FILE, 0, 0);
        print FILE, $top_of_buffer;
        print FILE, $new_stuff;
        print FILE, $bot_of_buffer;
        close(FILE);