Works fine for me:

#!/usr/bin/perl -l
use strict;
use warnings;
use Class::Struct;

struct A => {
  b => '%',
};

my $obj = new A;

print $obj->can('b');
$obj->b('foo', "bar");  # call setter
print $obj->b('foo');

use Data::Dumper;
print Dumper $obj;


__END__
CODE(0x8b5570)
bar
$VAR1 = bless( {
                 'A::b' => {
                             'foo' => 'bar'
                           }
               }, 'A' );
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(also works when I use $self->{ReferenceToA} in place of $obj)

What happens when you run this example?

Your example works for me too. Sorry. Apparently I took the error out of my code while reducing it to a simpler test case. My production code still exits with "Not a CODE Reference". I tried debugging it, but I do not understand why it crashes while evaluating the if-expression.

1357==>b if ($self->{TT}->call_category($key) != undef)
1358        {
1359            # Do something, never reached
1360        }
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use Data::Dumper;
print Dumper $key
$VAR1 = 'AN-AM-BA';
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print Dumper $self->{TT}->call_category
$VAR1 = {
          'DU-EX-TA' => 16,
          '*-CM-*' => 145,
          # Lots more, but not the value of $key.
          '*-VY-*' => 132
        };
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print Dumper $self->{TT}->call_category($key)
$VAR1 = undef;
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n
Not a CODE reference at ...
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Do you have any pointer what's wrong here?

# Lots more, but not the value of $key

Where do you set the hash entry (in case you expect it to be set)?

Anyhow, as you can tell from the $VAR1 = undef in

print Dumper $self->{TT}->call_category($key)
$VAR1 = undef;
[download]

when you try to fetch non-existent entries, you just get undef (not too surprising, actually).  However, it's not the accessor that causes the "Not a CODE reference" issue, but something later in code you haven't shown (otherwise the error would be thrown before Dumper gets the undef).  In other words, it might not have anything to do with Class::Struct.