You can't. You can use if the subroutine in local scope.

{
        local $subRef = sub {
        $var = shift;
         print "hello monks --- [$var] \n";
      };
        &callFun();
}

sub callFun     {

&$subRef(20);

}
[download]

OR

{
        $subRef = sub {
        $var = shift;
         print "hello monks --- [$var] \n";
      };
        
}
&$subRef;
[download]