I didn't find documentation on that module, but I've seen documentation for the C++ version. It accepted a file name, a file handle or a string containing an image in XPM format. What you are passing is none of those.

If the module accepts a Perl file handle (as opposed to a system file handle), the following will do the trick:

open(my $image_fh, '<', \$decoded);
my $image = Wx::Bitmap->new(
    Wx::Image->new( $image_fh, wxBITMAP_TYPE_ANY )
);
[download]

Requires Perl 5.8.0 or higher.