%INC is randomized nowadays, so sort it to get consistent results.
Also, more important, make sure $INC{$f} is really set, otherwise
we end up searching the filesystem root.

--- ./lib/Mail/Field.pm.orig	2013-07-15 12:06:12.000000000 +0000
+++ ./lib/Mail/Field.pm	2013-07-15 12:07:24.000000000 +0000
@@ -63,8 +63,9 @@ sub import
     }
 
     my($dir,$dir_sep);
-    foreach my $f (keys %INC)
+    foreach my $f (sort keys %INC)
     {   next if $f !~ /^Mail(\W)Field\W/i;
+        next unless $INC{$f};
         $dir_sep = $1;
         $dir = ($INC{$f} =~ /(.*Mail\W+Field)/i)[0] . $dir_sep;
         last;