High-order harmonic generation by the diffraction of an intense Laguerre-Gaussian (LG) laser beam through a small aperture is studied. It is found that the 2D peripheral electron dynamics at the boundary of the diffraction aperture can facilitate complex interplay between the spin and orbital angular momentum interaction, which leads to distinct selection rules for LG pulses with different polarization states. In particular, when the driver is linearly polarized, each harmonic beam contains multiple LG modes with different topological charges. The number of modes equals to the harmonic order and the total angular momentum is conserved during this process. A theory is derived and validated by simulations, which can predict the harmonic topological charges as well as their relative intensities for LG drivers with different polarization states. Our Letter provides fundamental insight into the behavior of light in nonlinear optics and paves the way toward high-intensity UV or x-ray pulses carrying controllable orbital angular momentum that can serve as versatile tools at frontiers of various scientific fields.

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• Angular momentum of light
• High intensity laser-plasma interactions
• Plasma surface high-order harmonics