You're right that it's an oversight. I didn't spot $bef[$mid] as an element of an array. For shame.

Perl--this'd be ActivePerl--had an interesting response to this script when the syntax was checked with '-w': it insisted that a right bracket was missing at line 24. Hmm. The version of the test that I'm running at the moment contains only 14 lines, of which the latter 7 are commented out.