You didn't specify which command you used. Try

#!/usr/bin/perl

use strict;
use warnings;
use IPC::Open3;

my @cmd = ( perl => (-e => 'print STDERR "x"x1024 for 1..100') );

local(*HIS_IN, *HIS_OUT, *HIS_ERR);
my $childpid = open3(*HIS_IN, *HIS_OUT, *HIS_ERR, @cmd);
close HIS_IN;
my(@outlines) = <HIS_OUT>;
waitpid($childpid, 0);
if ($?) {
    printf "Status is: %04X\n", $?;
}
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On linux, readline never returns.