Okay this is how I solved it, didn't manage to find the segfault reason though, just providing a quick way to do the job:

        my $mime_data = shift ;

        my ($orig, $x);
        $orig = Image::Magick->new ;
        $orig->BlobToImage($mime_data) ;
        my ($w, $h, $size, $format) = $orig->Ping(blob => $mime_data);
        $w = 1+int(($w-1)/8);
        $w *= 8;

        # Creating bg for composite image
        my $bg = Image::Magick->new ;
        $bg->Set(size=>$w."x$h",monochrome =>"True",magick =>"png");
        $bg->ReadImage('xc:white');
        $bg->Composite(image=>$orig, gravity=>"Center");
        my $blob = $bg->ImageToBlob()
        undef ($orig,$bg) ;

        my $final = Image::Magick->new ;
        $final->BlobToImage($blob) ;  
        $final->Set(magick => "wbmp") ;
        $blob = $final->ImageToBlob() ;
        undef $final ;
        return $blob ;
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