I'm assuming 'blubb' doesn't exist... If so, when you call open3() not being wrapped in an eval, the forked child dies with "open3: exec of blubb failed at ./720878.pl line 23", and you're reading that message via <$rdr> (because you passed in an undefined $err handle, the child's stdout and stderr are redirected to the same handle).

However, when you call open3() wrapped in an eval, it doesn't die with the above message, and thus your <$rdr> will wait forever. What makes things complicated is that - as the exec of blubb failed - the child lives on as a forked copy of the parent, which will start the next iteration (and another child process) after 10 seconds...

Moral of the story: don't wrap IPC::Open3::open3() in eval to catch exec errors.