my $var=3;
mysleep($var); # these two lines are fine

mysleep(3); # but this one causes an error!!!

sub mysleep {
           sleep($_[0]);
           $_[0]=10;
}

##</code><code>##

$y=substr("hello",10,1); 
foo($y);# this runs, but substr() generates warnings because the substr() is outside the string

foo( substr("hello",10,1) ); # this is a fatal error!