You might want to ask this on the Perl/Gtk2 maillist. IIRC, the Gtk2 pixbufs are limited to 24 bits data. Also there may be some clever bit-shift( which I'm unaware of) that will convert the 32 to 24 bit, for loading. From my notes:

  $pixbuf = Gtk2::Gdk::Pixbuf->new_from_data 
               ($pixels,  # packed image data in a scalar 
                'rgb',  # only 24- or 32-bit RGB are supported 
                0, # no alpha, data is 24-bit  
                8,     # only 8 bits per sample are supported  
                $w,  # in pixels  
                $h,  # in pixels 
                $w*3);  # number of *bytes* in each row
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