Well, let me point out some of the many problems with that script.

1. There is no use strict;
2. There is no use warnings;
3. The reserved words my and unless are in ALL CAPS
4. The functions lc and s/// are in ALL CAPS
5. There is a bareword (HTTP_REFERER)
6. The regex has some unnecessary grouping parentheses
7. The function ErrorHTML is not defined

Let me re-write it using some Perl Best Practices:

use strict;
use warnings;

# Just the domain name, no "www."
my $AuthorizedDomain = lc 'example.com';
my $FormDomain       = lc $ENV{'HTTP_REFERER'};
$FormDomain =~ s{
                    \A
                    https?://    # Strip off the protocol
                    (?:www\.)?   # Strip off any "www."
                    (.*?)        # Grab everything before...
                    /            # The first slash
                    .*           # And delete the rest
                    \Z
                }
                {
                    $1
                }msx;

if ($FormDomain ne $AuthorizedDomain) {
    ErrorHTML('Unauthorized access.');
}
[download]

Of course, for this to really work, the ErrorHTML function would have to be defined somewhere.

Update: Removed #5. Thanks, Your Mother.