Maybe I'm missing something, but it seems fairly easy to modify OP's code to check for a match: 
($left ? /ab(..)ef/ : /gh(..)jk/) or die("Bad data");

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This line is less readable in some ways than your suggested patterns, but it doesn't introduce any new variables.
Note: I got some results that looked funny until I replaced OP's (..) with (.*) in the regexen, watch out for that!
~dewey

In reply to Re^2: ?: and saving backreferences by dewey
in thread ?: and saving backreferences by throop

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