Gotcha! I was assuming you were getting the proper value into $datajoin somewhere in the code that wasn't included in the code sample.

if (!(param())) {
   # Do stuff
   # By processing data
   # put up web page
   # with formatted data 

   $datajoin =~ s/&/&/g;
   $datajoin =~ s/"/"/g;
   <form>
   <input type="hidden" name="datajoin" value="$datajoin">  
   <center><input type=submit value=Send name=S1>
   </center></form>
} else {
    # When it is print
    # Redirect to another web page
    if (param('S1') eq "Send") {
        $datajoin = param('datajoin');
        validate $datajoin
        open (BOBIN, ">>$datefile") || die "Cannot Open File $datefile
+ for writing: $!"; # Open Data file
        print BOBIN $datajoin;
        print redirect("...");
    }
}
[download]

Be sure to validate the data before saving it, because the user can change it! You might want to look into sessions. With sessions, only a session id is passed to the user (instead of $datajoin). The data is stored locally.