IO::Wrap does not do what you think it does. You want IO::String, IO::Scalar or open(..., \$text). The last one is best, but only works in Perl 5.8.0 and higher.

my $in_text = '...';
my $out_text;

open(my $in_fh, '<', \$in_text)
   or die("Unable to create input file handle from string: $!\n");

open(my $out_fh, '>', \$out_text)
   or die("Unable to create output file handle from string: $!\n");

my $decoded = $decoder->decode($in_fh, $out_fh);
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