simulate the error case when system() returns -1

t/error-http_download-exec-error.t

@@ -0,0 +1,26 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+use Test::More import => [ qw( done_testing like subtest ) ];
+use File::Temp qw( tempdir );
+
+BEGIN {
+    *CORE::GLOBAL::system = sub {
+        return $? = -1
+    };
+}
+
+use App::Perlbrew::Path;
+use App::Perlbrew::HTTP qw(http_download);
+
+local $ENV{PERLBREW_ROOT} = $App::perlbrew::PERLBREW_ROOT = tempdir( CLEANUP => 1 );
+
+subtest "The exit status code of curl", sub {
+    my $error = http_download( "https://example.com/whatever.tar.gz",
+        App::Perlbrew::Path->new($App::perlbrew::PERLBREW_ROOT)->child("whatever.tar.gz") );
+
+    like $error, qr/^ERROR: Failed to execute the command/;
+};
+
+done_testing;