use IPC::Run3 qw(run3);
my @cmd = ('perl', 'timestamp.pl');
run3 \@cmd, undef, \&out, \&err;
sub out {
  print time(), " out: $_\n";
}
sub err {
  print time(), " err: $_\n";
}
__END__
# The problem is that run3 buffers the output.
1215162770 out: 1215162760 info
1215162770 out: 1215162760 info
1215162770 out: 1215162761 info
1215162770 out: 1215162761 info
1215162770 out: 1215162761 info
1215162770 out: 1215162761 info
1215162770 out: 1215162762 info
1215162770 out: 1215162762 info
1215162770 out: 1215162762 info
1215162770 out: 1215162763 info