  We perform 3D relativistic ideal MHD simulations to study the collisions
between high-$\sigma$ (Poynting-flux-dominated) blobs which contain both
poloidal and toroidal magnetic field components. This is meant to mimic the
interactions inside a highly variable Poynting-flux-dominated jet. We discover
a significant electromagnetic field (EMF) energy dissipation with an Alfv\'enic
rate with the efficiency around 35\%. Detailed analyses show that this
dissipation is mostly facilitated by the collision-induced magnetic
reconnection. Additional resolution and parameter studies show a robust result
that the relative EMF energy dissipation efficiency is nearly independent of
the numerical resolution or most physical parameters in the relevant parameter
range. The reconnection outflows in our simulation can potentially form the
multi-orientation relativistic mini-jets as needed for several analytical
models. We also find a linear relationship between the $\sigma$ values before
and after the major EMF energy dissipation process. Our results give support to
the proposed astrophysical models that invoke significant magnetic energy
dissipation in Poynting-flux-dominated jets, such as the internal
collision-induced magnetic reconnection and turbulence (ICMART) model for GRBs,
and reconnection triggered mini-jets model for AGNs.
