This is a fairly simple question, i hope, the code below was suggested by ferrency to whom i am grateful, except that i am too ignorent to make it work.
my only restrictions are $username needs to have 3-16 letters/numbers and nothing else. 
#!/usr/local/bin/perl -wT
use strict;
use CGI;
...
 $username =~ s/\W//g;
    if ($username =~ /(\w{2,16})/)
      {
        $username =~ $1;
        $username =~ tr/[A-Z]/[a-z]/;
      }
 else
      {
        #since username isnt able to be safe, it is empty
        $username ="";
      }

`/bin/echo $username`;

[download]
Insecure dependency in `` while running with -T switch is my error... it seems as though the statement should be safe and untainting right? so what have i done wrong?

In reply to a terribly basic question showing my inept skills with regular expressions by jcpunk

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