The use attributes line is there for two reasons:

1. Loads attributes.pm if it isn't loaded.
2. Regardless of if it was loaded, calls attributes->import( ... ) with the params passed.

I'm willing to bet that what happens is that the breakpoint is set on the "sub a", but that function isn't executed. Instead, it is replaced by the one declared by attributes. Unfortunately, the attributes heavy lifting code is within the Perl source, so it's a little less accessible to one such as I.