Comment # 2 on bug 911585 from Michal Hocko

(In reply to Michal Hocko from comment #1)
> I am not a filesystem expert nor familiar with perl but it might be possible
> that the mtime was not synced yet because perl didn't flush (msync) the file
> yet and system does that automatically only after certain (configurable)
> time.

Hmm, but that shouldn't matter because the stat should be checking the inode
via dentry/inode cache so it doesn't care what is on the disk. Maybe perl does
some internal caching without calling the kernel. What is the strace output of
your test?