You could simply exclude the opening parenthesis from the page capture and the closing parenthesis from the section capture like so:

/^([^(]+)\(([^)]*)/
[download]

Here's a test with your four examples:

$ perl -E '
    my @manpages = qw{
        ftpd(8)
        dhcp-config(5)
        Cache::Cache(3)
        Tk::widget::demo(3)
    };

    for my $line (@manpages) {
        my ($page , $section) = $line =~ /^([^(]+)\(([^)]*)/;
        say "page[$page] section[$section]";
    }
'
page[ftpd] section[8]
page[dhcp-config] section[5]
page[Cache::Cache] section[3]
page[Tk::widget::demo] section[3]
[download]