my $un = shift;
unless( $un ~= s/\A([_A-Za-z][_A-Za-z0-9]+)\Z/$1/ ){
 # Log/Warn(L4/5); #?
 exit &ask_user_to_resubmit_request; 
}
$un = lc($un);
...
unless($<){..}

##</code><code>##

#uid check
unless($< == 0){ # Log attempt and exit; 
 }else{ print "Sue! How do you do?\nWe've been rooting for you" }

#untaint and foldcase user data
my $un = shift;
unless( $un ~= s/\A([A-Za-z]\W+)\Z/$1/ ){
 # Log attempt 
 exit &ask_user_to_resubmit_request; 
}
$un = lc($un);

#carry on
if(defined(getpwnam $un)){ .. }