You might want to check that the file GUESTBOOK is actually opened. Since you didn't post that bit of code, I'm presuming it should look something like this:

open(GUESTBOOK, ">>", $guestbook) || die "Could not open guestbook\n";

If your filehandle isn't open, you may be printing to nowhere.

Further, where are you getting %form_data from? The code you posted shows you declaring it, then suddenly expecting things to exist by magic. Here's how you might do it:

use CGI;
my $cgi = CGI->new();

my %form_data = %{$cgi->Vars};
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Also, watch out! use strict. I think you're making a mistake when you do this:

$form_data{$name}

Don't you mean this?

$form_data{name}

With strict and warnings, the variable reference should cause a complaint, since $name is presumably undeclared.