use strict;
use warnings;

our $exchange = "before (set by caller)";

my $other_file  = "this_other_thing.pl";
my ($out, $err) = redir_file( $other_file );

#-- proof of re-direction and import/export
print "Caller: GOT from STDOUT ($other_file): ",
  join("\n\tOUT>  ", "", split( /\n/, $out) ), "\n";
print "Caller: GOT from STDERR ($other_file): ",
  join("\n\tERR>  ", "", split( /\n/, $err) ), "\n";
print "Caller: Exchange : $exchange\n";


sub redir_file {
  my $do_filename = shift;

  #-- we DUP the current STREAMS and then we close the original streams...
  open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
  open(my $olderr, ">&STDERR")     or die "Can't dup STDERR: $!";
  close STDOUT;
  close STDERR;


  #-- now, we re-open the streams with redirection into strings...
  my $redir_stdout;
  my $redir_stderr;
  open( STDOUT, '>', \$redir_stdout ) or die "Cannot redirect STDOUT to string: $!";
  open( STDERR, '>', \$redir_stderr ) or die "Cannot redirect STDERR to string: $!";


  #-- this is like "do"... and exactly as insecure...
  my $do_file_content = qx{cat $do_filename};    # e.g. use Slurp...
  my $read            = eval $do_file_content;   # =8-O
  #... now $exchange might have been updated

  #-- now, we restore the previous streams...
  open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
  open(STDERR, ">&", $olderr) or die "Can't dup \$olderr: $!";

  return ($redir_stdout, $redir_stderr);
}
 

##</code><code>##

#!/usr/bin/env perl 
use strict;
use warnings;
use utf8;

our $exchange;

print STDOUT  "PRG: Hello World!\n";
print STDERR  "PRG: Hello Err!\n";
print         "PRG: Exchange: ", ( $exchange // '(nothing)'), "\n";

$exchange = "after (called program)";


##</code><code>##

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