Thanks for that clarification, but you still haven't answered Corion's equally good objection++ above, namely:

The rule is each array contains a sorted (ascending) sequence of non-contiguous, unique integers, but my @a3 = ( 205, 206, 315 ); contains the two contiguous numbers 205 and 206.

Should we be reasoning based on your descripition of the problem, or on your supplied dataset?