index(substr($str[$_], 0, $n), $str[$_ + 1], 0) != 0

##</code><code>##

Benchmark: running eq_index, eq_substr, each for at least 10 CPU seconds...
  eq_index: 14 wallclock secs (10.08 usr +  0.00 sys = 10.08 CPU) @  3.08/s (n=31)
 eq_substr: 16 wallclock secs (10.30 usr +  0.00 sys = 10.30 CPU) @  3.01/s (n=31)
            Rate eq_substr  eq_index
eq_substr 3.01/s        --       -2%
eq_index  3.08/s        2%        --