From perldiag (you can get these explanations for error messages automatically using use diagnostics;):

Deep recursion on subroutine "%s"

(W recursion) This subroutine has called itself (directly or indirectly) 100 times more than it has returned. This probably indicates an infinite recur­sion, unless you're writing strange benchmark pro­grams, in which case it indicates something else.

Update: changed 'use warnings' to 'use diagnostics', which is what I meant, as jsprat pointed out later...