I'm not clear on what you mean by "called from the html page using a image reference", and this may not apply since you're using "a simple socket server", but I use the following environment variables from CGI.pm.

$ENV{'REMOTE_HOST'}
$ENV{'REMOTE_ADDR'}
$ENV{'HTTP_USER_AGENT'}
[download]

If this is what you're looking for, more info can be found at http://stein.cshl.org/WWW/software/CGI/.

No mater how you extract this info, it's trivially spoofed, so you shouldn't base any security on what browsers tell you about themselves.
    cheers,
    ybiC