You can open the subprocess as a filehandle and then read from that filehandle. See open for the simplest case and IPC::Open2IPC::Run2 and IPC::Open3IPC::Run3 for the more complex cases.

my $cmd = '/engine/monitor';
my $pid = open my $fh, '-|', $cmd
    or die "Couldn't launch [$cmd]: $!/$?";
for( 1..10) {
    print scalar <$fh>;
};
close $fh; # kills the child when it next tries to write
kill $pid;
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