It doesn't look any levels down. If you:

use lib "/tmp";
use Foo::Bar;
[download]

... then Perl will use "/tmp/Foo/Bar.pm", but not "/tmp/x/Foo/Bar.pm".

The exception for your CPU architecture name (in your case "x86_64-linux-thread-multi") is due to some trickery by use lib which automatically detects those directories if they exist (and look like proper locations containing architecture-specific libraries), and unshifts two entries onto @INC instead of the normal one.