Thanks for the replies, I figured it out now, I think.

Saying

my My $my
[download]

indeed tells the compiler that $my should be a My object. This information is not used for type-checking, however (neither at run-time nor at compile time). It is currently only used to make use of the fields pragma, which lets you define your class' fields:

# compile error
$my->{no_such_field} ++;
[download]

In addition to checking the spelling of field names, you get the improved performance of having the hash access turned into an array/pseudo-hash access (On the other hand, pseudo-hashes seem to be evil, the docs warn about their imminent removal from Perl).

use strict;
{
    package My;
    use fields qw[ one  ];
    sub new {
       return fields::new(shift);
    }
}
my  My $my = new My;
$my->{one}++;
[download]

b-deparses to

package My;
sub BEGIN {
    use strict 'refs';
    require fields;
    do {
        'fields'->import('one')
    };
}
sub new {
    use strict 'refs';
    return fields::new(shift @_);
}
package main;
use strict 'refs';
{;};
my $my = 'My'->new;
++$$my[1];  ##  <--- array access
[download]

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