use CGI qw(:standard);

my $file  = 'foo.txt';
my $lines = read_file($file);

print header(), start_form(),
   textarea(
      -name     => 'head',
      -value    => $lines,
      -rows     => 6,
      -cols     => 60,
      -wrap     => 'soft',
      -override => 1,
   ), p(submit());

my $head = strip(param('head'));

print end_form();

if ($head) {
   write_file($file,$head);
}

sub read_file {
   my $file = shift;
   open(FILE,$file) or die "$file not readable: $!";
   my @lines = <FILE>;
   close(FILE);
   return join('',@lines);
}

sub write_file {
   my ($file,$new_head) = @_;
   print STDERR "adding $new_head";
   open (FILE,'>',$file) or die "$file not writeable";
   flock (FILE, 2)       or die "$file not lockable";
   print FILE $new_head;
   close(FILE);
}

sub strip {
   my $param = shift;
   return '' unless $param;

   $param =~ s/^\s+//g;
   $param =~ s/\s+$//g;
   return $param;
}