Are you sure $stream is set to what you think it is?

If you don't need to capture output and just want it to go to stdout, use system(@array):

# Not sure if you are trying to pass literal '\t','\n' or tab and newl
+ine characters...assuming tab and newline
system(cleartool => 'lsco', '-r', '-brt', $stream, '-fmt', "%En\t%u\n"
+);
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If you want to capture stdout, open a file handle to the command:

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: $!";
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