sub by_val {
   my ($foo) = @_;
   print "Foo = $foo\n";
   $foo = 10;             # only 'foo' belonging to this sub is modified
}

sub by_ref {
    my ($foo_ref) = @_;
    print "Foo = $$foo_ref\n"; # pring value in 'bar'
    $$foo_ref = 10;            # will modify 'bar'
}

my $bar = 5;

by_val($bar);
print "bar = $bar\n";
by_ref(\$bar);        # $bar may no be modified inside the sub.
print "bar = $bar\n";