Where I'm stuck is that I will need to determine the ip address of the machine from which the login attempt failed.

On a non-production machine that you don't care about try the following very evil code:

#!/usr/bin/perl -w
use CGI qw/ :standard /;

my $cgi=new CGI;
print header,start_html;

print h1("Environment");
print table(
Tr(th("Environment Variable"),th("Value")),
map { Tr(td($_),td($ENV{$_})) } sort keys %ENV );
print hr;

print h1('CGI Varialbes');
print table (
Tr(th("parameter"),th("parameter value")),
map { Tr(td($_),td($cgi->param($_))) } sort $cgi->param ) ;

print end_html;
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I call this evil because besides what you are looking for is some information that you really don't want to leak to the "public."

Of particular interest in your case is the following key and value that are found in the %ENV hash:

REMOTE_ADDR     68.37.227.52
[download]

There's other information there as well but, this was what you specifically asked for.

Update: (GAH!) ikegami got his post out there before I hit "submit" Oh well, the code I put up there is good stuff anyway if you want to "play."