We demonstrate a scheme to produce coherent extreme ultraviolet and soft x-ray radiation with subpetahertz temporal helicity modulation, based on the high harmonic generation from current-carrying orbitals driven by intense linearly polarized laser fields. It is found that the electronic angular momentum of the orbitals oscillates periodically in femtosecond scale in the driving field. This so-far undescribed phenomenon is qualitatively interpreted and attributed to the laser-induced energy shift of orbitals. Consequently, the polarization of the harmonic radiation switches periodically in the temporal domain between left and right elliptical polarizations, and the frequency of the helicity modulation reaches subpetahertz. By varying the intensity of the laser field, the modulation frequency can be continuously controlled. This light source will serve as a potential tool to detect and manipulate the ultrafast dynamics in magnetic materials and chiral media.