I get a segfault as well (even if I change $a and $b to $main::a and $main::b). The stack trace shows that it fails in pp_sort.c (sortcv), apparently with NULL pointers for a and b:

#0  sortcv (my_perl=0x9a4c7a0, a=0x0, b=0x0) at pp_sort.c:1591
1591    pp_sort.c: No such file or directory.
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Running it under the debugger worked a few times, and then crashed with a SEGV, at the same location.

perl 5.8.3 i386-linux-thread-multi.

Michael