The issue of how to account for the effects of execution of code done at compile time is a deep one, with serious consequences for parsing Perl, compiling it, etc. The decision to allow that functionality imposes implicit limitations on Perl.

However my thinking is that since these techniques are becoming more widely used on CPAN, support for doing them are moving into Perl's core, and the plans for Perl 6 are to improve Perl's support for this kind of functionality, I think that trying to ignore this issue is not wise, and will become increasingly not wise as time goes on.

The moral is that you can only get 2 out of 3 of the following desirable features:

1. Easy external parsing and manipulation of code.
2. Support for macros.
3. Syntactic sugar.

Lisp has 1 and 2. TCL 1 and 3. Perl is in the process of adding 2 to 3.