|There's more than one way to do things|
Nscd provides cacheing for the passwd(5) ... databases through standard libc interfaces, such as getpwnam(3), getpwuid(3), ... and others. Each cache has a separate TTL (time-to-live) for its data; modifying the local database (/etc/passwd, and so forth) causes the cache to become invalidated within fifteen seconds.
So one possibility is that after changing the LDAP directory, waiting 15 seconds will give you the right data. Another possibility is that touching /etc/passwd (or another file configured in nscd.conf) after making the change will cause the cache to be invalidated, at least after 15 seconds.
You could also consider getting the UID directly from LDAP.
And, the reason your run of getent avoids the cache is that it doesn't call getpwnam or getpwuid, but instead goes through the entire passwd database, probably with getpwent. You could probably get the same effect without running a seperate utility by just calling that function repeatedly from within Perl.
It looks like you can control how long nscd caches data in /etc/nscd.conf; see nscd.conf(5).
As to whether it's a bug, that depends on what you expect from your system. Caching is a trade-off of speed for accuracy; if you don't want that trade-off, you can avoid running nscd, or configure it for very short timeouts.