Run3 apparently doesn't do concurrency. IPC:Run can be made to do what you want. This works:
#!/usr/bin/perl
# timestamp.pl
while(time - $^T<10){
select undef,undef,undef, 0.1;
if ( rand()<0.5) {
print STDERR time(), " warn\n";
} else {
print time(), " info\n"; }
}
print "end\n";
__END__
use IPC::Run qw ( start pump finish timeout );
my $in;
my $out;
my @cmd = qw( perl timestamp.pl);
## Build the harness, open all pipes, and launch the subprocesses
my $h = start \@cmd, \$in, \$out, \$out ;
$in = "" ;
## Now do I/O. start() does no I/O.
pump $h while length $in ; ## Wait for all input to go
## Now do some more I/O.
while (1){
pump $h until $out =~ s/(.+)\n$//gm ;
last if ("end" eq $1);
print time, " : $1\n";
}
## Clean up
finish $h or die "cat returned $?" ;
__END__
1215226647 : 1215226647 info
1215226647 : 1215226647 info
1215226647 : 1215226647 warn
1215226647 : 1215226647 info
1215226647 : 1215226647 warn
1215226647 : 1215226647 warn
1215226647 : 1215226647 info
1215226647 : 1215226647 info
1215226647 : 1215226647 info
1215226648 : 1215226648 warn
s//----->\t/;$~="JAPH";s//\r<$~~/;{s|~$~-|-~$~|||s
|-$~~|$~~-|||s,<$~~,<~$~,,s,~$~>,$~~>,,
$|=1,select$,,$,,$,,1e-1;print;redo}