my $code = <<END_CODE;
owns(merlyn,gold).
owns(ovid,books).
owns(kudra,gold).
valuable(perl).
valuable(gold).
valuable(corinna).
query :-
    owns(X,Y), valuable(Y),
    writef('["%t", "%t"], ',[X,Y]).
main :-
    writef('[ '),
    bagof(_,query,_),
    writef(']\n').
END_CODE

my $tmpfile = "deleteme.pro";
open(TMP,">$tmpfile"); print TMP $code; close(TMP);
my $result = `pl -q -f deleteme.pro -t main`;
unlink($tmpfile);
print $result;