The most robust way would be to start the program as the root user, loop through the passwd file with getpwent, switch to each of the UIDs, and try to open the file or opendir the directory. That will work with all kinds of ACLs and other oddities your version of Unix may include.

If you know your system only uses standard Unix stuff, you can just open the bottommost directory, make a hash of users who have access to it, then remove one directory component at a time with File::Basename's dirname function. Make sure all of the users in your hash have at least execute permission on that parent directory, and if not delete them from the hash. When you've checked the root filesystem, you're done, and whoever is in the hash can access that directory.

You may also have to deal with symlinks. For some help on that, see 346747.

Hmm, I was thinking of working from root out, trimming the hash, since I've got a directory tree I'm anotating. Going from the file/directory down to the root would work for the general "Why can't this user access this file question" better. I'll try it both ways and see which is faster.

If you just want to check occasionally whether a particular user can access a file or directory, I have a script for that here:

http://www.suspectclass.com/~sgifford/permcheck/permcheck

Be careful. A user needs execute permission on a directory to pass through, read permission to view contents. opendir will succeed on a directory where you have '--x' permissions but you will obviously not have access to directory contents. Make sure a component of your solution actually tries a read on each directory.

Update:

I was not clear, opendir will fail on a directory that doesn't have it's 'r' bit set (assuming non-root). However if I open a directory say /homes/insensate/allx/freeforallWhere  allx is  --x--x--x and freeforall is  rwxrwxrwx the opendir will succeed.

My point is to not let an assumption that access to a given directory implies access to parent directories stand.

That's not how my copy of perl is behaving:

$ ls -ld /tmp/test
drwx--x--x    2 root     root         1024 May  4 15:55 /tmp/test
$ perl -e 'opendir(D,"/tmp/test") or die "opendir failed: $!\n";'
opendir failed: Permission denied
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for user in `awk -F: '{print $1}' /etc/passwd`; do 
  echo $user; 
  su - $user -c "ls -l /path/to/dir/" >/dev/null 2>&1;  
  echo $?; 
done
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Yeah I wrote it on the command line :) You might need to tweak for your system, skipping system accounts for example (awk -F: '$3>100{print $1}' /etc/passwd usually does it).