I had a similar issue before our last audit from AIB when my boss asked if he could have maintenance put a new set of racks against a wall in the warehouse due to lack of space. First, there is no inherent risk associated with not having an 18" white strip as long as you have it addressed in your MCS or master cleaning schedule. That is, a new SOP for cleaning and inspecting may need to be developed depending on what is stored on the racks (potential to attract pests and or allergens). Just go ahead and remove the old white line and bolt down the feet. Then, caulk around the feet to prevent pest harborage. So, to make a long story longer, I okayed the racks being next to the wall and not a word was mentioned during the audit. I think the 18" rule should still apply to the ceiling and at least 6" off the floor for food safety concerns. I hope this helps.
I'll agree with others in that there is no GFSI recognized standard that lists a specific perimeter length, but the inherent risk is that a rat or mouse will feel whatever is stored there and run over to it instead of along the wall. Rats feel with their adorable lil whiskers, so if *stuff* is in their pathway, they crawl over to it and hide rather than scurrying along the wall and running into a trap. You don't need 18 inches for that - but you might need 18 inches to access the traps - unless you want to put a weight limit on people who can check them. Maybe people can safely and comfortably walk the perimeter with less than 18, but if you're larger it becomes an issue.
Space is an issue in my factory as well. For the walls which are "shared"/internal within the company we don't require it because we are controlling all of the walls around it. In areas where we can not get 18 in we put a piece of the racking across the bottom to stop pallets from going in and it's cheaper than angle iron.