# Not sure if you are trying to pass literal '\t','\n' or tab and newline characters...assuming tab and newline
system(cleartool => 'lsco', '-r', '-brt', $stream, '-fmt', "%En\t%u\n");

##</code><code>##

open(my $fh, "-|",
  cleartool => 'lsco', '-r', '-brt', $stream, '-fmt', "%En\t%u\n") or die "Err: $!";
while (<$fh>) {
  print "OUTPUT: $_";
}
close $fh or die "Error: $!";