  Perturbation theory of a large class of scalar field theories in $d<4$ can be
shown to be Borel resummable using arguments based on Lefschetz thimbles. As an
example we study in detail the $\lambda \phi^4$ theory in two dimensions in the
$Z_2$ symmetric phase. We extend the results for the perturbative expansion of
several quantities up to N$^8$LO and show how the behavior of the theory at
strong coupling can be recovered successfully using known resummation
techniques. In particular, we compute the vacuum energy and the mass gap for
values of the coupling up to the critical point, where the theory becomes
gapless and lies in the same universality class of the 2d Ising model. Several
properties of the critical point are determined and agree with known exact
expressions. The results are in very good agreement (and with comparable
precision) with those obtained by other non-perturbative approaches, such as
lattice simulations and Hamiltonian truncation methods.
