One of the $r->print()'s declared abilities is to pass reference in place of scalar value (to prevent "unneeded copying of large strings").
But when I use Apache::Filter and pass my output to another handler (module) I can't use this feature. IMHO that's because Apache::Filter->print uses print STDOUT @_ in place of print STDOUT ref $_ ? $$_ : $_ foreach (@_) Am I right or what? Dirty hack on Apache::Filter sources solved the problem. P.S. $Apache::Filter::VERSION == '1.019'
