Now that you've had a bunch of nice recursive solutions here's an iterative one

my %hash = (
    foo => {
        bar => {
            baz  => 'one',
            quux => 'two',
        },
    },
    field => 'value',
    this  => {
            that => undef,
    },
);

my @stack = \%hash;
my @path;
while(@stack) {
    my($k,$v);
    unless( ($k, $v) = each %{ $stack[0] } ) {
      shift @stack;
      pop   @path;
      next;
    }
    
    if(ref $v eq 'HASH') {
        unshift @stack => $v;
        push    @path  => $k;
    } else {
        print   join('/' => @path, $k), " => ",
              ( defined $v ? $v : "undef" ), "\n";
    }
}

__output__

foo/bar/quux => two
foo/bar/baz => one
this/that => undef
field => value
[download]

Not a recommendation by any means, but just another way to do it. It's essentially a recursive approach unrolled which is done by maintaining your own stack and probably quite handy when you're optimizing recursive algorithms (see. Unrolling recursion for more info).
HTH

_________
broquaint