#! perl -slw
use strict;

open GIF, '<:raw', $ARGV[ 0 ] or die $!;
my $gif = do{ local $/; <GIF> };
close GIF;

## Extract the colour table
my @GCT = unpack '(A3)256', substr $gif, 13, 256 * 3;

## SWap the colours end to end
@GCT = reverse @GCT;

## put the colour table back
substr( $gif, 13, 256*3 ) = pack '(A3)256', @GCT;

open OUT, '>:raw', 'modified.gif' or die $!;
print OUT $gif;
close OUT;