my %foo = ('a' => 1);
my_sub(\%foo);

sub my_sub {
 my $foo_ref = shift;

 #Now I want to do something here to name %bar as pointing to %foo:
 #my %bar ????? $foo_ref;

 #such that 
 print $bar{a};
 #prints '1'

 #and if I were to
 $bar{b} = 2;
};

print $foo{b};
#prints '2'