The username should be in the environment variable REMOTE_USER.

Rather than play around with CGI.pm and other things, for these situations I use a test script that prints out the complete environment. Look at the lowest level that you can. :)

#!/usr/bin/perl

print "Content-type: text/plain\n\n";

foreach my $key ( sort keys %ENV )
    {
    printf "%-25s %s\n", $key, $ENV{$key};
    }
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