Finally, a graphics system that lets you at the values in the GIF, rather than pretranslating them back to colours. Thanks for pointing me in the right direction. However, I thought it better to go with this:

#! perl -slw
use strict;
use GD;

my $img = GD::Image->new( $ARGV[0] ) or die $!;

my %palette;
my $total= 0;
my( $x, $y ) = $img->getBounds;
for my $yi ( 0 .. $y-1 ) {
    ++$palette{ $img->getPixel( $_, $yi ) } for 0 .. $x-1;
}

for my $i ( sort{ $a <=> $b } keys %palette ) {
    $total += $palette{ $i };
    printf "index:%3d (%02x:%02x:%02x) n:%d\n",
        $i, $img->rgb( $i ), $palette{ $i };
}
print( " $x * $y = ", $x*$y, " =?= $total\n");
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M.