@keywordList = ('john', 'john.smith', 'john.smith@mail.com');
$combinedExpression = combine(@keywordList);
# The combined expression looks something like this:
# (john(?:\.smith(?:\@mail\.com)?)?)
$searchText = "john's username is john.smith and his email address is john.smith@mail.com";
while $searchText =~ /$combinedExpression/g {
   print "$1\n";
}