My first thought was to generate a pair of hashes, like jmcnamara did. It's the standard Perl mechanism for translating a set of strings to some other encoding.

But in this case left and right are purely arbitrary labels. left123 is as likely to occur on the left as on the right.

So instead of placing the tags into two hashes, %L_tags and %R_tags, I would simply use one hash, %tags.