Try testing it from the command line.

perl -MCrypt::PasswdMD5 -wle'print unix_md5_crypt("password")'
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You should not get any errors when you run the above; if you do, then your installation of Crypt::PasswdMD5 may be messed up.

If the above test succeeds, then I'd suggest expanding the tests in your CGI. Ensure that your program contains the following at the top (you may need to adjust the level of spices and the cooking time somewhat to suit your own palate):

#!/usr/bin/perl -wT
use strict;
use CGI::Carp qw/fatalsToBrowser warningsToBrowser/;
use CGI qw/:standard/;
$|++;
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If you still haven't solved the problem at that point, you'll at least have plenty of ammunition for troubleshooting it - or for reporting the exact error here rather than just an "http 500 error".

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells