RDF:Query: langMatches wrongly parsed or emitted

[nichtich]

use v5.14;
use RDF::Query;
my $q = 'SELECT * WHERE { ?s ?p ?l . FILTER langMatches(lang(?l), "en") . }';
say RDF::Query->new($q)->as_sparql;

Output

SELECT * WHERE {
    ?s ?p ?l .
    FILTER( LANG(LANG(?l), "en") ) .
}

instead of

SELECT * WHERE {
    ?s ?p ?l .
    FILTER( LANGMATCHES(LANG(?l), "en") ) .
}