You can still go into the character screen before boosting your stats. Just click cancel on the perk window, and it will appear next time you try to open your character screen. Every time the perk window comes up, it will refresh the choices based on your stats. This works with both the positive and negative affects of drugs. If you take mentats to get bonus perception to meet the Sniper perk requirements, you can do that. If you rest a few hours and lose the bonus perception, and suppose your original perception was 8, then you wouldn't be able to get Sniper because the perk system does not care what your original stats are, only your current stats.