I had the same problem recently while writing a perlTk script. I found out it helped me to use the perl tie command.

tie(*STDERR, 'Tk::Text', $text); 
tie(*STDOUT, 'Tk::Text', $text); 
$SIG{'__WARN__'} = sub { print STDERR @_ };
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You could also use the module Tie-STDERR. It has an option for:

use Tie::STDERR \$append_to_scalar;
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good luck
Tie-bag
Sure there's more than one way, but one just needs one anyway - Teabag