# ... $image is your GD::Image for output

my $gd = $image->gd;
my @attrs = unpack 'nnnCV', substr( $gd, 0, 11, '' );
my $len = length( $gd ) / 4 * 3;
my $bmp = pack 'a2 V V V l< l< l< v v V V l< l< V V',
    'BM', $len + 54, 0, 54, 40, $attrs[1], $attrs[2], 1, 24, $len, 0, 0, 0, 0;

$bmp .= join'', unpack '(xaaa)*', $gd;

## Now print $bmp to a file xxx.bmp and convert; or maybe pipe directly to an image converter for conversion.