Interesting. However, given the documentation on my system this mistake is not difficult to understand... From perlsub

Overriding may be done only by importing the name from a module--ordin
+ary
predeclaration isn't good enough. However, the use subs pragma lets yo
+u, 
in effect, predeclare subs via the import syntax, and these names may 
+then 
override built-in ones:


    use subs 'chdir', 'chroot', 'chmod', 'chown';
    chdir $somewhere;
    sub chdir { ... }
[download]

But thanks for the clarification.