Open a pipe to your child process then using fcntl do non-blocking reads (sysread) from the pipe. Then you can poll for 3 seconds and if nothing is read from the pipe, kill the child process

Just a tongue-tied, twisted, earth-bound misfit. -- Pink Floyd

Use IO::Select on a pipe.

use IO::Select;

my $s = new IO::Select();
my $pid = open PH, "arbitrary command|" or die "arbitrary command: $!"
+;
$s->add( \*PH );

my @ready = $s->can_read( 3000 );
if (@ready == 0) {
  # nothing to read, kill the child and close the pipe
  kill( $pid);
  close PH;
}
else {
  while (<PH>) {
    # do something with the output
  }
}
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dmm