$hash = thaw builds a new structure and replaces the reference in $hash with a reference to that structure. It does not change $href or what it points to.

The following illustrates memory contents after each statement:

sub do_stuff {

          +-----+  +------------------+
$href --->| a: --->| status: "OK"     |
          +-----+  +------------------+


    my $hash = shift;

          +-----+  +------------------+
$href --->| a: --->| status: "OK"     |
$hash /   +-----+  +------------------+


    my $hash2 = freeze($hash);

          +-----+  +------------------+
$href --->| a: --->| status: "OK"     |
$hash /   +-----+  +------------------+

         +--------------------------------+
$hash2 = |  Frozen image of               |
         |  +-----+  +------------------+ |
         |  | a: --->| status: "OK"     | |
         |  +-----+  +------------------+ |
         +--------------------------------+


    $hash->{'a'}->{'status'} = "NOT OK";

          +-----+  +------------------+
$href --->| a: --->| status: "NOT OK" |
$hash /   +-----+  +------------------+

         +--------------------------------+
$hash2 = |  Frozen image of               |
         |  +-----+  +------------------+ |
         |  | a: --->| status: "OK"     | |
         |  +-----+  +------------------+ |
         +--------------------------------+


    $hash = thaw($hash2);

          +-----+  +------------------+
$href --->| a: --->| status: "NOT OK" |
          +-----+  +------------------+

         +--------------------------------+
$hash2 = |  Frozen image of               |
         |  +-----+  +------------------+ |
         |  | a: --->| status: "OK"     | |
         |  +-----+  +------------------+ |
         +--------------------------------+

          +-----+  +------------------+
$hash --->| a: --->| status: "OK"     |
          +-----+  +------------------+


    print "IN SUB: " . $hash->{'a'}->{'status'} . "\n";
    return 0;
}
[download]

The simplest and safest solution is to not change the structure in the first place by working on a copy of it.

sub do_stuff {
    my $hash = dclone(shift);
    $hash->{'a'}->{'status'} = "NOT OK";
    print "IN SUB: " . $hash->{'a'}->{'status'} . "\n";
}
[download]

sub do_stuff {

          +-----+  +------------------+
$href --->| a: --->| status: "OK"     |
          +-----+  +------------------+


    my $hash = dclone(shift);

          +-----+  +------------------+
$href --->| a: --->| status: "OK"     |
          +-----+  +------------------+

          +-----+  +------------------+
$hash --->| a: --->| status: "OK"     |
          +-----+  +------------------+


    $hash->{'a'}->{'status'} = "NOT OK";

          +-----+  +------------------+
$href --->| a: --->| status: "OK"     |
          +-----+  +------------------+

          +-----+  +------------------+
$hash --->| a: --->| status: "NOT OK" |
          +-----+  +------------------+


    print "IN SUB: " . $hash->{'a'}->{'status'} . "\n";
    return 0;
}
[download]