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Oil canning or sheet rolling and deformations is very common when perforating. When a border is used around the perimeter of the part the challenges are compounded. As the sheet expands and tension is released the sheet starts to warp adding complicated secondary work and in extreme cases scrap as the sheet will pull out of the clamps.

Three things I have done will not necessarily eliminate this, but greatly reduce the undesirable normal effect of perforating.

1. Always use the absolute correct die clearance. Any cheating on die clearance will have adverse effects. Measure the sheet thickness. Just because it states it is 20 gauge does not mean it measures .032. If the steel supplier is providing steel at .036 and calling it 20 gauge, then run 20% die clearance off of the .036 and not the .032. Yes .001 does make a difference.
2. Slow the ram speed down. Productivity is key, but any secondary operation will absorb that gain. Slowing the ram speed down reduces the “hammer” effect of the punch when it meets the die. The ideal is to cut with the punch and not hammer the sheet before the cutting starts. Any kid has taken a ball peen hammer to sheet metal to know what it will do.
3. Sharpen more frequently. This plays off of the above. Cutting is key, not pushing; especially if the perforations are small rounds. If you normally sharpen once every eight hours or 250,000 cycles, pull the punch and dust it off at four hours or 100,000 cycles. A sharp punch will pay huge dividends down the road. Removing just .001-.002 thousands is irrelevant from a tooling cost, but produces significant gains concerning output.

Always uses a metal stripper when available and making sure the sheet is flat, never use the punch to flatten the sheet when perforating.