Are you doing your testing in Windows or Unix? select (the underlying system call used by IO::Select) only works for sockets in Windows.

"$em_command &" is silly, complicates things, and might even cause problems. Simply $em_command is needed.

open3 throws an exception on error. It doesn't return, much less with a string matching /^open3:/.

$handle contains the handle, but you pass $rh to sysread.