Your child probably filled up the STDERR pipe and blocked trying to add more. You have a race condition.

• The child is blocked printing to STDERR until the parent empties some of the child's STDERR.
• The parent is blocked reading from child's STDOUT until the child sends to STDOUT.

If you don't need to capture the child's STDERR, this problem can be avoided by sending the child's STDERR to the parent's STDERR or to a file (such as /dev/null).

open3(*HIS_IN, *HIS_OUT, '>&STDERR', ...)
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open(local *NULL, '>', '/dev/null') or die;
open3(*HIS_IN, *HIS_OUT, '>&NULL', ...)
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If you need to capture the child's STDERR, you'd do better to use a higher-level module such as IPC::Run3 or IPC::Run. Otherwise, you'll need to use select or threads or non-blocking IO.