select STDERR;
$| = 1;
select STDOUT;
$| = 1;

open(CMD, 'perl b.pl 2>&1|') or die $!;
open(LOGFILE, '>', 'b.log') or die $!;

my $input;
while(sysread(CMD, $input, 1024)) {
    print $input;
    print LOGFILE $input;
}
close(CMD);