If providing implementation is part of the API, you might want to have better control about user errors

you could do something like (untested)

sub my_meth {
    my $self = shift;
    my $cmd  = $self->{api_meth};
    unless ( defined $cmd ) {
        # default
    } else {
         eval { $cmd->(@_) }     
            or raise_error();
    }
}
[download]

Of course this will be slower than your suggestions...

TIMTOWTDI and YMMV :)