If you have a use line for Crypt::PasswdMD5, and it's not installed, things should die right there.

If you have a require line for Crypt::PasswdMD, and your program makes it past that OK, that also proves that you have it installed, but you'll still get the error you're getting. In that case, change the "require" to "use" instead.

If your program does not ever "use" or "require" Crypt::PasswdMD5, you need to add that.

use Crypt::PasswdMD5;
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If that doesn't work, I'm guessing you have an installation problem.

Is the web application using the same perl (and perl library), @INC settings, PERL5LIB environment variable, etc as your command line?

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