Yes, it has to be one regex... basically I'm using some third-party software (which I don't want to have to hack) that wants me to provide it with the contents of a regular expression in a config variable so that it can use it later for substitution. Something like this:

  # IN the config file
  $regexp = '@example.com';
  $replacement = '@other.com';

  # Many places nestled in the code
  $mail = s/$regexp/$replacement;
[download]

I think I found what I was looking for. Thanks to all.