  We investigate a perturbation of a scalar field model (called here the
signum-Gordon model) with the potential $V(f)=|f|$. The perturbation
generalizes the signum-Gordon model to the signum-Klein-Gordon model i.e. to
the case $V(f)=|f|-{1/2}\lambda f^2$, where $\lambda$ is a small parameter.
Such a generalization breaks the scaling symmetry of the signum-Gordon model.
In this paper we concentrate on solutions for self-similar initial data. Such
data are particulary useful for identification of the effects caused by the
term that breaks the scaling symmetry. We have found that the behaviour of the
solutions is quite interesting - they escape and return periodically to the
self-similar initial data.
