On perl 5.14.2 (the only version I've got on this 'ere Linux machine, but I assume it's the same on 5.8), -w and friends boil down to stat(2) calls:

$ strace perl -e '-w "/etc"'
...
stat("/etc", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
...
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Perl doesn't know about filesystems and automount, it just relies on the OS to Do The Right Thing. If you write a little snippet of C code that just stat()s a directory, then I expect you'll find that it behaves the same.