# STDOUT reader
open PIPE_1, "outer.pl 2>/dev/null |" 

# STDERR reader
open PIPE_2, "outer.pl 1>/dev/null |" 

# outer.pl
print STDERR "stderr!$/";
print STDOUT "stdout!$/";

...
print while <PIPE_1>; # stdout! 
print while <PIPE_2>; # stderr!
...