a-zA-Z0-9_ == \w
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I'd use an array mapped to a hash to check existance of user - easier maintenance

my %sysuser = map { $_ => 1 } qw(administrator accounts support postma
+ster webmaster spam-admin  technical billing sales purchase buy misus
+e assistance mail virus-admin manager usenet hostmaster);

if (defined $sysuser{$username}) {
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If you're loading CGI, you might as well use it for output:

$w->p('Please Accept The Terms And Conditions.');

print $w->header.
       $message;
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etc.

From a style view, (personally), I'd use @error to store errors:

my @errors=();
...
push @errors, 'Password Must Be Between 6-30 Chars With No Symbols.';
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then check with

if (@errors) {
    print $w->header.
          join '', map { $q->p($_) } @errors;
    exit(0);
}

# you only need one cgi object
$w->redirect();
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Note, most people use $q(uery) or $cgi for the CGI object you might want to do the same, especially when you want to read up CGI tutorials here. It might help :)

Note - above is untested for exact syntax, but that's the general idea. And yes, use SSL!

But overall, I think you've done a pretty good job if you're new to Perl :)

.02

cLive ;-)