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Perineural invasion (PNI) is a distinct route of cancer spread associated with neuropathic pain, local recurrence, and poor survival across many solid tumours. Increasing evidence shows that PNI is not only a structural pattern of invasion but also a dynamic biological process involving neurodegeneration, nociceptor sensitisation, and marked local immunosuppression. This narrative review synthesises experimental, translational, and clinical data on the molecular, neurological, and immunological mechanisms of PNI in solid malignancies. PNI arises through complex crosstalk between tumour cells, Schwann cells, macrophages, fibroblasts, and neurotrophic pathways, leading to peripheral nerve remodelling, axonal degeneration, and abnormal regeneration. These changes promote neuropathic pain through ion-channel dysregulation, neurotrophin-driven sensitisation, and pathological neuroplasticity. At the same time, PNI creates an immunosuppressive microenvironment enriched in Tregs, M2 macrophages, and myeloid-derived suppressor cells, shaped by cholinergic, adrenergic, and neuropeptidergic signalling, which may contribute to immune exclusion and resistance to immunotherapy. We propose that PNI should be understood as a neuro-immuno-metabolic process and that recognising the PNI-pain-immunosuppression triad may support the development of targeted neuroprotective, analgesic, and immunomodulatory therapies.