Your program does two things. It prints HTML output (a form you need to fill in) and then it appends to a data file. But you should only be appending to the data file after the form has been submitted. And your FORM tag has a very weird URL: "http://localhost/newuser.cgi?action=newuser?first?last" doesn't make a lot of sense. You probably just want "http://localhost/newuser.cgi".
Here is a simple program:
#!/usr/bin/perl
use CGI;
use CGI::Carp qw( fatalsToBrowser );
use strict;
use warnings;
my $query = CGI->new;
# if we got here from a form being submitted,
# do the adduser() stuff
adduser($query) if $query->param;
print << "END_HTML";
<html>
<body>
<form method="post" action="thisprogram.cgi">
Username: <input type="text" name="user"><br>
First name: <input type="text" name="fname"><br>
Last name: <input type="text" name="lname"><br>
Password: <input type="password" name="pass"><br>
<input type="submit" value=" Add User ">
</form>
</body>
</html>
END_HTML
sub add_user {
my ($q) = @_;
open PASSWD, ">> $passwd_file"
or die "can't append to $passwd_file: $!";
print PASSWD join(":" =>
$q->param('first'), $q->param('last'), $q->param('user'), $q->para
+m('pass')
), "\n";
close PASSWD;
}
