Hmm...that still doesn't work for me. I'm on a Win32 system though, so I tried:

set PERL5LIB=perl5lib;%PERL5LIB%
perl -Mifdef=DEBUGGING foo.pl
[download]

I also tried changing foo.pl like so:

# ./foo.pl:
use lib './perl5lib';
use ifdef qw/DEBUGGING/;
use Bar;

=begin DEBUGGING
die "limbo";
=cut

die "outer death";
__END__
[download]

Here, the 'use lib' clearly comes before 'use ifdef', but still no luck.

I should also note that, unexpectedly, this doesn't output 'limbo' either (when run as 'perl foo.pl'). However, that is another issue that is, I believe, unrelated to 'use lib'. In the thread I originally referenced, it indicates that "this pragma is only intended to be called from the commandline" anyway.