You can probably use IO::Scalar to tie STDERR to a scalar variable. Something like this (untested):

use IO::Scalar;
my $stderr;
tie *STDERR, 'IO::Scalar', \$stderr;
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Now anything written to STDERR will end up in $stderr.

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg