# ...snip...

# untaint parameters
for( keys %params )
{
    # !!!TODO!!! check 'ref' line for subtle bugs
    ( display_message( $messages{error} ) && exit )
        unless ref($valid_params{$_}) eq 'Regexp';
    if( $params{$_} =~ /$valid_params{$_}/ )
    {
        $params{$_} = $1;
    }
    else
    {
        display_message( $messages{error} ) && exit;
    }
}

##</code><code>##

my $userfile = get_userfile( $config, $params{username} );

##</code><code>##

sub get_userfile
{
    my ( $config, $username ) = ( shift, shift );

# add only this line: still tainted
# ( $config->{ users } ) = ( $config->{ users } =~ /^(.+)$/ );

# add only this line: untainted
# ( $username ) = ( $username =~ /^(.+)$/ );

    $config->{ users } . $username;
}