So, if someone wanted to call the same method twice on the same object, you've managed to completely remove that ability.

I guess you could do something like:

sub myfunction {
    my $self = get_object( shift );

    ... # whatever's function specific, and not related to
        # creation of the object
}

sub get_object {
    my $self = shift;
    if ( !ref $self ) { return $self->new() } # a string?
    if ( UNIVERSAL::isa( $self, __PACKAGE__ ) { return $self }
    return __PACKAGE__->new();
}

sub new {
   ...
}
[download]

But, it doesn't seem like a very clean interface to me ... I mean, how can we tell what arguments should've been part of the object initialization, and what were arguments to the method call?

Personally, I'm a user of SOAP::Lite, but I have a feeling that its history, having been created when SOAP was basically just a thin wrapper around XML RPC, was a problem, as almost all of the examples you find are these sorts of calls. I don't know that there is a good 'best practice' in how to deal with passing objects using SOAP::Lite. (if there is, I'd love to know about it myself)