sub validURL {
    my ($self, $url) = @_;
    my $req = new HTTP::Request HEAD => $url;
    my $res = $self->request($req);    
    my $content = $res->content;
    return 0 unless $res->is_success;
    return 0 if $content =~ /the page you have requested cannot be fou
+nd/i;
    return 1;
}
[download]

Note that I have changed the request method from POST to HEAD - The POST method will not be allowed for most URLs (thereby generating your false-negative results) and while this could be changed to a GET request, the HEAD request method will be more successful for all "valid" URLs, irrelevant of the preferred request method.

perl -e 's&&rob@cowsnet.com.au&&&split/[@.]/&&s&.com.&_&&&print'