It's a funny bug in DBD::Google. If you look at the parser you'll see that @default_columns contains URL, but a few lines later it gets changed in this for loop:

for my $dc (@default_columns) {
    $dc =~ s/([A-Z])/_\l$1/g;
    $allowed_columns{$dc} = 1;
}
[download]

so that URL becomes _u_r_l.

I am not going to test it, but I'd change the loop to:

for my $dc (@default_columns) {
    my $t;
    ($t=$dc) =~ s/([A-Z])/_\l$1/g;
    $allowed_columns{$t} = 1;
}
[download]

-- 
        dakkar - Mobilis in mobile