Most systems implement passwords that only allow letters and numbers. In this case, a decent taint check would be:

        my ($checked) = $submitted =~ m/^([a-zA-Z0-9])$/;
        if (!defined $checked){
          croak "Invalid name or  password.\n"
        }
[download]

Almost right.
Most systems allow users to enter passwords that matches with

/[a-zA-Z_0-9\(\)\[\]\{\}\+\=\-\\\/\*]{$MinPwLen,$MaxPwLen}/
[download]

I would use something like this to untaint a password.

Where:

$MinPwLen is the minimal password size in chars.

$MaxPwLen is the maximal (if any maximal is needed password size in chars.

Note: Don't forget to document and tell your users about what characters are valid to compose a password. This is very important.

Note 2:Keep this kind of information away from evil internet script-kidies, they can use this to narrow a dictionary-like attack and break into your system faster.

May the gods bless you

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Just Another Perl Monk