  In-plane anisotropy of electrical resistivity was studied in samples of the
hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ in the composition range $0.21 \leq x
\leq 0.26$ where anisotropy changes sign. Low-temperature ($\sim$20~K)
irradiation with relativistic 2.5 MeV electrons was used to control the level
of disorder and residual resistivity of the samples. Modification of the
stress-detwinning technique enabled measurements of the same samples before and
after irradiation, leading to conclusion of anisotropic character of
predominantly inelastic scattering processes. Our main finding is that the
resistivity anisotropy is of the same sign irrespective of residual
resistivity, and remains the same in the orthorhombic $C_2$ phase above the
re-entrant tetragonal transition. Unusual $T$-linear dependence of the
anisotropy $\Delta \rho \equiv \rho_a(T)-\rho_b(T)$ is found in pristine
samples with $x=$0.213 and $x=$0.219, without similar signatures in either
$\rho_a(T)$ or $\rho_b(T)$. We show that this feature can be reproduced by a
phenomenological model of R.~M.~Fernandes {\it et al.} Phys. Rev. Lett. {\bf
107},217002 (2011). We speculate that onset of fluctuations of nematic order on
approaching the instability towards the re-entrant tetragonal phase contributes
to this unusual dependence.
