use Fcntl qw(:flock);
use Symbol;

my @lines =
  (
   qq{Complete documentation for Perl, including FAQ lists,\n},
   qq{should be found on this system using `man perl' or\n},
   qq{`perldoc perl'. If you have access to the Internet, point\n},
   qq{your browser at http://www.perl.com/, the Perl Home Page.\n},
  );

my $fh = gensym;
open $fh, "+>>/tmp/foo" or die $!;

# start critical section
flock $fh, LOCK_EX;
seek $fh, 0, 0;
push @lines, <$fh>;

seek $fh, 0, 0;
truncate $fh, 0;
print $fh @lines;
close $fh; # close unlocks the file
# end critical section