You should change perform.ini using the mIRC interface (Alt-O, Connect, Options, Perform).

If for some reason you want to be able to edit the code from outside mIRC, it might be smarter to work with an on connect event, which basically does the same thing as the perform.ini, but mIRC detects changes made to the script.ini file. (Still, modifying files externally that are in use by mIRC is not a smart thing.)

Code:
on *:connect: {
  if $network == A {
    dostuffhere
  }
  elseif $network == B {
    dostuffhere
  }
  elseif $network == C {
    dostuffhere
  }
  else {
    dostuffhere
  }
}

Learning something new every day.