I worked out a slighlty modified version of the great and really useful code above. I added support for 3 hex colors and changed default behavior to exit if no $rgb is matched. 
use strict;

my($rgb) = $ENV{PATH_INFO} =~ m|/([0-9A-F]+)(?:\.gif)?$|i;

grep {length $rgb == $_} (3, 6) || exit;

$rgb = join '', map { $_ x 2 } $rgb =~ /./g if length( $rgb ) == 3;

my $gif = pack("H*",
               '47494638396101000100B30000' . $rgb .
               'FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF' .
               'FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF' .
               'FFFFFFFFFFFFFFFFFFFFFFFFFF2C0000' .
               '0000010001000004021044003B'
              );

binmode(STDOUT);
print "Content-type: image/gif\r\n";
print "Content-length: ", length($gif), "\r\n";
print "\r\n";
print $gif;

[download]
$|=$_='1g2i1u1l2i4e2n0k',map{print"\7",chop;select$,,$,,$,,$_/7}m{..}g

In reply to Re: On-demand single-pixel GIFs by giulienk
in thread On-demand single-pixel GIFs by dws

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