Ah, I knew it: code runs with result (almost) as expected on 1st attempt: no errors, no typos even. Then something _must_ be wrong with it:). Because of those "glitches" I said "almost" and "didn't investigate", though I should have :)

First thing, "copy" method doesn't copy gif tags (bug in Imager?), $trans_idx was undefined, mask always blank.

However, "compose" method apparently works, even if source image is converted to 3 channel RGB (no alpha) with "to_rgb8", and mask attribute not supplied at all. Statement in fragment further below could be replaced with

$backdrop-> rubthrough( src => $_-> to_rgb8, 
                        tx => $left, ty => $top );
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and animation would still work. Where and why Imager gets transparency information in this case -- I don't know. I'd expect the result be the same as with "paste" method (you can try it if you wish -- that's where there are no masks, no transparency).

Further, I should have checked for offsets of gif frames -- they are not always zero. Whole "if else" fragment can be replaced with the following (though mask may be unnecessary(?), I'm not omitting it):

    if ( $backdrop ) {              # not 1st?
        my $mask = $_-> copy;
        my @mask_plt = map NC( 255, 255, 255 ), $_-> getcolors;
        my $trans_idx = $_-> tags( name => 'gif_trans_index' );
        my $left      = $_-> tags( name => 'gif_left' );
        my $top       = $_-> tags( name => 'gif_top' );

        $mask_plt[ $trans_idx ] = NC( 0, 0, 0 ) 
            if defined $trans_idx;
        $mask-> setcolors( colors => \@mask_plt );
        $backdrop-> compose( src => $_, mask => $mask, 
                             tx => $left, ty => $top );
    }
    else {
        $backdrop = $_-> to_rgb8
    }
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