Having just solved a very similar problem, I would suggest the following pseudocode. What it boils down to is using the query object to pass parameters to the other runmodes.

sub join {
  # This is the 'join' runmode
  my $self = shift;
  my $q = $self->query;

  # It submits to the verify runmode
  if ($self->query->param('error')) {
    # We just tried to submit, but had error(s); print them
  }
  # Print the rest of the form, etc
}

sub verify {
  # 'verify' runmode
  my $self = shift;
  my $q = $self->query;

  # Do error checking
  unless (@errors) {
    # This is how we 'redirect' without redirecting.
    $q->param('username',$username);
    $q->param('password',$password);
    return $self->create;
  } else {
    $q->param(-name => 'error', -value => \@errors);
    return $self->join;
  }
}

sub create {
  # 'create' runmode
  my $self = shift;
  my $q = $self->query;
  # Create the account
  return $self->viewprofile;
}

sub viewprofile {
  # 'viewprofile' runmode
  # ..uses $q->param('username') which is either supplised by the user
+, or
  # by the code in 'verify', which was passed to 'create' and then on
  # to us.
}
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