use {{ ... effectively turning the line in the next debugger step into a comment.

I don't think this has the desired effect of commenting out the lines. Rather, it runs the debugger command "#" (which does nothing of interest) before running the line itself. Even if you'd use pre-Perl actions (i.e. <), they would still be executed separately, rather than being textually merged with the line(s) in question (as would be required to comment out).

Try skipping over the $i++, for example... the subsequent print " $i\n" will still print the incremented value (e.g. 3, not 2, on the first iteration).

An example application of pre-debugger commands would be setting "{ l" to get a listing of the next few lines on every prompt.