As of 5.37.7 when an @INC hook is executed the index of the @INC array that holds the hook will be localized into the $INC variable. When the hook returns the integer successor of its value will be used to determine the next index in @INC that will be checked, thus if it is set to -1 (or undef) the traversal over the @INC array will be restarted from its beginning.
Normally traversal through the @INC array is from beginning to end (0 .. $#INC), and if the @INC array is modified by the hook the iterator may be left in a state where newly added entries are skipped. Changing this value allows an @INC hook to rewrite the @INC array and tell Perl where to continue afterwards. See "require" in perlfunc for details on @INC hooks.