The asn_db object in the default profile contains a sources object, and since it works, that should match the implementation.

{
    "asn_db" : {
        "style" : "Cymru",
        "sources" : {
            "Cymru" : [ "asnlookup.zonemaster.net", "asn.cymru.com" ],
            "RIPE" : [ "riswhois.ripe.net" ]
        }
    }
}

The documentation, however, says that sources should be an array:

=head2 asn_db.sources
An arrayref of domain names when asn_db.style is set to C<"Cymru"> or whois
servers when asn_db.style is set to C<"RIPE">. Normally only the first item
in the list will be used, the rest are backups in case the earlier ones don't 
work.
Default C<"asnlookup.zonemaster.net">.

(From Zonemaster/Engine/Profile.pm)

Either the documentation or the implementation should be updated.

In that Perl module there is also no documentation for the asn_db object.