As Ted says, it's the antihalation dye in the film. It's meant to be destroyed by the alakali in a developer, but presumably, highly diluted perceptol doesn't do the job.(posted 8335 days ago)
Verichrome Pan supposedly has two coats of emulsion, one fast and one slow in speed, which gives it its good latitude (and poor resolution!). This also makes the film very thick, by today's standards, and any dye in the emulsion will take longer to diffuse out, or be bleached by the developer.