If you're trying to re-define WWW::Mechanize's _make_request method, maybe

...
package WWW::Mechanize;

sub _make_request {
    my($self) = shift;
    my $req = $self->SUPER::request(@_);
    print Dumper $req;
    return $req
}
...
[download]

is what you want ? Another possible problem might be that you suppose that WWW::Mechanize's STDOUT (where Dumper would write to) is something that comes in handy to you. That isn't really granted ;)