Please the <code> tags around your code. (It's custom to PerlMonks.) It makes it (more) readable, preserves whitespace, handles HTML escaping, wraps lines if needed, and provides a mechanism to view the code unwrapped.

One problem I see is that you use <FH> instead of sysread. See the warning at the bottom of the documentation for select.

Another problem I see if that it only prints "STDOUT: " or "STDERR: " for every chunk, not necessarily every line. You need to split on newlines what you get.

I never figured out the use of some methods in IO::Select. What's the use of can_read if you can't check for errors? The proper usage seems to be:

use IO::Select;

$selector = IO::Select->new();
$selector->add(*CMD_ERR, *CMD_OUT);

while ($selector->count()) {
   my ($readable, undef, $error) = 
      IO::Select::select($selector, undef, $selector);

   $selector->remove($_)
      foreach @$error;

   foreach my $fh (@$readable) {
      ...
   }
)
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I don't know what you mean by "left hanging".