cover an extra error case when download target already exist.

t/error-http_download-param-validation.t

@@ -0,0 +1,21 @@
+use strict;
+use Test::More;
+use Test::Exception;
+use File::Temp 'tempdir';
+use IO::All;
+
+use App::Perlbrew::HTTP qw(http_user_agent_program http_get http_download);
+
+subtest "http_download: dies when when the download target already exists" => sub {
+    my $dir = tempdir( CLEANUP => 1 );
+    my $output = "$dir/whatever";
+
+    io($output)->print("so");
+
+    my $error;
+    throws_ok {
+        $error = http_download("https://install.perlbrew.pl", $output);
+    } qr(^ERROR: The download target < \Q$output\E > already exists\.$);
+};
+
+done_testing;