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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.