The light leak might show as worse on frames where you have the bellows folded out for longer times, have the film sitting in one spot longer, or are exposing the camera to brighter light while the film sits stationary for a longer time. If it's always the first frame, it might also be your loading technique, but that's hard to diagnose without seeing the film. Is it only in the picture area, or also on the edges of the film? Especially heavy edge fogging is usually loading problems, or perhaps a leak in the camera back seals. In the image area only is likely to be bellows leakage.
I shot a roll with small pinholes in my Isolette bellows, but didn't see any fogging, as it was dim and overcast, and I was careful about advancing and closing the bellows. I'm sure I could have induced fogging by leaving the bellows open for long periods in bright sun.
Jurgen Kreckel, certo6.com, could do a bellows replacement and lens and shutter CLA for you if you don't want to DIY. His prices are reasonable, and he'll give you an estimate by email. My Agfa Isolette I was about $100 for CLA and bellows replacement. Don't know if the Perkeo II would be the same, but it's a similar camera. He's in Pennsylvania. I was happy with his service on my camera, which was quickly and well done.
I'm a little late, and echoing the recommendation by Will S by the time I typed this. You gotta be fast on APUG.