lib.pl
-------------
package lib;
use vars '%common'; 

%common = ( 'username' => " " );

sub untaint_username
  {
    my $tainted = shift;

    # patern match letters and numbers for 1-16 characters (inclusive)
    $tainted =~ s/\W//g;  # remove all non word characters
    if ($tainted =~ /\A([a-zA-Z0-9]{1-16})\z/i)
      {    return $1;   }   # behold a username
    else {print "failure\n";}
  }

1;
##</code><code>##

-----------------
prog.pl
-----------------
#!/usr/bin/perl -wT
use strict;

require './lib.pl';

$lib::common{username}="unixhelp";

$lib::common{username}=lib::untaint_username($lib::common{username});