Here's a helpful explanation from perlop of this odd behavior of the glob operator:

       A glob evaluates its (embedded) argument only when it is
       starting a new list.  All values must be read before it
       will start over.  In a list context this isn't important,
       because you automatically get them all anyway.  In scalar
       context, however, the operator returns the next value each
       time it is called, or a undef value if you've just run
       out. As for filehandles an automatic defined is generated
       when the glob occurs in the test part of a while or for -
       because legal glob returns (e.g. a file called 0) would
       otherwise terminate the loop.  Again, undef is returned
       only once.  So if you're expecting a single value from a
       glob, it is much better to say
 
           ($file) = <blurch*>;
 
       than
 
           $file = <blurch*>;
 
       because the latter will alternate between returning a
       filename and returning FALSE.
[download]

So, even though you've changed the argument to this glob() in your loop, the glob doesn't notice that the argument has changed until it finishes with the argument it got the first time it was called. And it takes two iterations for the glob to finish that argument: once to get the matching file and once to realize that there are no more files.

If you use the doc's suggested solution your problem should disappear!