This line writes into unallocated memory:
https://metacpan.org/source/JNW/Search-WuManber-0.25/WuManber.xs#L83
Since you have i++ <= n_patterns, at the end of the loop i = n_patterns + 2, because i is incremented after taking the value and failing the test.
You can check this error using
perl -I blib/lib -I blib/arch -MTest::Valgrind t/01-demo.t
after installing Test::Valgrind from CPAN. This is with the untarred CPAN distribution, not with the altered one which I posted to github.
Also, the arguments to calloc should be (n, size), not (size, n) at this line:
https://metacpan.org/source/JNW/Search-WuManber-0.25/WuManber.xs#L63
RT for rt.cpan.org