The problem is that you're using File::stat, but you're using 'stat' as if it was the built-in version. Either switch to 'stat->mtime' in your invocation or get rid of 'use File::stat;' at the top of the script; either one should fix you up just fine.

-- 
Education is not the filling of a pail, but the lighting of a fire.
 -- W. B. Yeats