open2 mixes STDERR into STDOUT, its arguments are "backwards", and it just ends up calling open3 anyway. I avoid it.

use strict;
use warnings;

use IPC::Open3 qw( open3 );

my $file = $ARGV[0];
open(local *TO_CHAIN, '<', $file)
   or die("Can't open \"$file\": $!\n");

my $gz_pid = open3('<TO_CHAIN, local *PIPE, '>STDERR', 'gzip -dc');
my $mc_pid = open3('<PIPE', local *FR_CHAIN, '>STDERR', 'mycommand');

while (<FR_CHAIN>) {
   print;
}
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