Unfortunately raw_datachannels doesn't follow raw_storechannels, so if you supply one, you probably need to supply the other, this also lets you switch to pack "C*" to pack the data as bytes. I don't have your data, so I can't check this does exactly what you want.

my $img = Imager->new(
    type  => 'raw',
    xsize => 256,
    ysize => 256,
    data  => pack('C*', @data),
    raw_interleave    => 0,
    raw_storechannels => 1,
    raw_datachannels => 1,
    model => 'gray'
);
$img->write(file=>'data.png');
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