The <> operator is super fussy due to being overloaded with ancient glob() craziness. It only works if you put in a simple scalar, not any kind of expression. This works:

  my $fh = $pids{$key}{'fh'};
  while (<$fh>){print $_}
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This kind of thing will work fine until your sub-processes need to produce more than 4k of data (or whatever your buffer size is). Then the sub-procs will block when they print() and won't work in parallel while you're collecting results. To fix this you need to switch to using something like IO::Select or EV to pull data from each process as it's ready.

-sam