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use strict;
use warnings;

...
  return ($redir_stdout, $redir_stderr);
}

#!/usr/bin/env perl 
use strict;
...
print         "PRG: Exchange: ", ( $exchange // '(nothing)'), "\n";

$exchange = "after (called program)";

Caller: GOT from STDOUT (this_other_thing.pl): 
        OUT>  PRG: Hello World!
...
        ERR>  PRG: Hello Err!
Caller: Exchange : after (called program)