That looks like Base64 encoding. There are clues that can help you identify those lines (64 chars wide, except maybe the last, the chars are only those allowed by Base64 (no space), etc...) but since you have "BEGIN ENCRYPTED PRIVATE KEY", I'm guessing you might also have an END. If that's the case, the better solution might be to ignore all the lines between those two tokens. Using the "from..to" version of the .. operator, this could be something like:
next LINE if ($line=~/BEGIN ENCRYPTED/)..($line=~/END ENCRYPTED/);
