if($str_check = undef){do stuff};{return ('0');}else{return($str_check);}

##</code><code>##

if($str_check = undef) ...

##</code><code>##

if (! defined $str_check) ...

##</code><code>##

if($str_check = undef){do_stuff()};

##</code><code>##

return $str_check if defined $str_check;
return '0';