perl -i.bak -lpe 's/PERL/perl/g' file

##</code><code>##

        {
          local ($^I, @ARGV) = ('.bak', 'file');
          while (<>) { 
            s/PERL/perl/g;
            print;
          }
        }

##</code><code>##

        tie @lines, 'Tie::File', 'file' or die ...;
        for (@lines) {
          s/PERL/perl/g;
        }
        untie @lines;

##</code><code>##

        for (@lines) {
          if (/<!-- insert here -->/) {
            $_ .= $text;
            last;
          }
        }

##</code><code>##

        unshift @lines, $new;   # Or add more than one record

##</code><code>##

        open DB, "< $database" or die ...;

        while (<DB>) {
          chomp;
          my ($username) = split /:/;
          $db{$username} = $_;
        }

        sub lookup {
          my $user = shift;
          return $db{$username};
        }

##</code><code>##

        tie @DB, 'Tie::File', $database or die ...;
        for (@DB) {
          my ($username) = split /:/, $_;
          $recno{$username} = $lineno++;
        }

        sub lookup {
          my $username = shift;
          return $DB[$recno{$username}];
        }

##</code><code>##

        sub replace_data {
          my ($username, $new_data) = @_;
          my $recno = $recno{$username};
          if (defined $recno) {  # Update existing user 
            $DB[$recno] = $new_data;
          } else {
            push @DB, $new_data;  # Add new user at the end
          }
        }

        sub update_password {
          my ($username, $new_password) = @_;
          my $crypted_password = crypt($new_password, random_salt());
          my @data = split /:/, lookup($username);
          $data[1] = $crypted_password;
          replace_data($username, join(':', @data));
        }