open my $FH, "the_file";   # open for reading
flock($FH, LOCK_SH);       # lock it
my @lines = <$FH>;         # read it
open $FH, ">the_file";     # open for writing: LOCK DROPPED.
# Now someone else picks up the lock and finds an empty
# file.  When they're finished,
flock($FH, LOCK_EX);       # lock it again
push @lines, "new line\n"; # process the lines
print $FH for (@lines);    # write them out
close $FH;                 # close and drop lock