# You already have DKIM key for mydomain.com
dkim_key('mydomain.com', 'dkim',       '/var/db/dkim/mydomain.com.pem');

# Cover your subdomains in @dkim_signature_options_bysender_maps= ():
@dkim_signature_options_bysender_maps = ( {
    ...[OMIT OTHER SETTINGS HERE]...

    # Cover subdomain "dev.mydomain.com".
    'dev.mydomain.com'                => { d => 'mydomain.com' },

    # OR, cover ALL subdomains with one key.
    '.mydomain.com'                => { d => 'mydomain.com' },
});