In this respect tying is no different from passing any other hash around. If you want a subroutine to modify the original you have to pass a reference. As it stands now you're just copying the contents of the hash around.

my %hash = qw(foo 1  bar 2);
foo(%hash);
bar(\%hash);
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The foo() subroutine gets a list, ('foo', 1, 'bar', 2). It can copy this list into a hash, but changes to that hash have no effect on %hash. bar() gets a reference to the original hash; changes to the reference affect %hash in the outer code.

Similarly, if you want to maintain the tie when you pass your hash out of hash_init(), you need to return a reference. Otherwise you're just copying the contents of your hash into another, non-tied, hash. You could also pass a hash reference for it to tie, i.e. hash_init(\%hash)

See perldoc perlreftut, perldoc perlref, and Beginning Perl's Chapter on References.