The behaviors of backticks are described in `STRING` in perlop. Backticks only return once the external program has finished execution - otherwise, how would perl know it had returned to you the entire output? Therefore, you can't use a $SIG{ALRM} to control it - the child process has control of the thread.

For the simple case, you can follow roboticus's suggestion to use fork.(Update: See cdarke's comment below) You might also consider threads (core) or POE for more complicated queuing behaviors. If you are feeling adventurous, you can combine a piped open with select or $SIG{ALRM} - see Using open() for IPC some some basics.