It would appear to be 5.8.8 that is the deciding factor as to whether it segfaults or not. Having installed the latest versions threads:v1.62 and Event:v1.09; I get no trap from your code under 5.8.6(AS811), but I do under 5.8.8(AS817).

However, if I do not nest the threads stuff inside the Event callback, no trap occurs either. HTH.

Update: I finally got the segfault to occur under the auspices of a debugger.

The trap is occurring in ev.c:187, when the if statement attempts to access WvACTIVE(wa). That doesn't explain why it happens, but it might give somebody a starting point.

Disassembly of Function Event.dll!pe_event_postCB (0x00363D57)
  ;*******************************************************************
+*************
  SYM:pe_event_postCB <Void>
  ; ev.c - Line 183
  ; static void pe_event_postCB(pe_cbframe *fp) {
  0x363D57: PUSH     EBP                                  
  0x363D58: MOV      EBP,ESP                              
  0x363D5A: SUB      ESP,0xC                              
  ; ev.c - Line 184
  ;     pe_event *ev = fp->ev;
  0x363D5D: MOV      EAX,DWORD PTR [EBP+0x8]              ; ARG:fp <pe
+_cbframe *>
  0x363D60: MOV      EAX,DWORD PTR [EAX]                  
  0x363D62: MOV      DWORD PTR [EBP-0x4],EAX              ; VAR:ev <pe
+_event *>
  ; ev.c - Line 185
  ;     pe_watcher *wa = ev->up;
  0x363D65: MOV      EAX,DWORD PTR [EBP-0x4]              ; VAR:ev <pe
+_event *>
  0x363D68: MOV      EAX,DWORD PTR [EAX+0x8]              
  0x363D6B: MOV      DWORD PTR [EBP-0x8],EAX              ; VAR:wa <pe
+_watcher *>
  ; ev.c - Line 186
  ;     --CurCBFrame;
  0x363D6E: MOV      EAX,DWORD PTR [0x10012020]           
  0x363D73: DEC      EAX                                  
  0x363D74: MOV      DWORD PTR [0x10012020],EAX           
  ; ev.c - Line 187
  ;     if (WaACTIVE(wa) && WaINVOKE1(wa) && WaREPEAT(wa))
  0x363D79: MOV      EAX,DWORD PTR [EBP-0x8]              ; VAR:wa <pe
+_watcher *>
+ 0x363D7C: MOV      EAX,DWORD PTR [EAX+0x20]             ; TRAP OCCUR
+S HERE <<<<<<<<<<<<<<<<<
  0x363D7F: AND      EAX,0x1                              
  0x363D82: TEST     EAX,EAX                              
  0x363D84: JZ       0x363DB0                             ; (*+0x2C) 
  0x363D86: MOV      EAX,DWORD PTR [EBP-0x8]              ; VAR:wa <pe
+_watcher *>
  0x363D89: MOV      EAX,DWORD PTR [EAX+0x20]             
  0x363D8C: AND      EAX,0x4000                           
  0x363D91: TEST     EAX,EAX                              
  0x363D93: JZ       0x363DB0                             ; (*+0x1D) 
  0x363D95: MOV      EAX,DWORD PTR [EBP-0x8]              ; VAR:wa <pe
+_watcher *>
  0x363D98: MOV      EAX,DWORD PTR [EAX+0x20]             
  0x363D9B: AND      EAX,0x2000                           
  0x363DA0: TEST     EAX,EAX                              
  0x363DA2: JZ       0x363DB0                             ; (*+0xE) 
  ; ev.c - Line 188
  ;     pe_watcher_on(wa, 1);
  0x363DA4: PUSH     0x1                                  
  0x363DA6: PUSH     DWORD PTR [EBP-0x8]                  ; VAR:wa <pe
+_watcher *>
  0x363DA9: CALL     0x3642EA                             ; SYM:pe_wat
+cher_on <Char *>
  0x363DAE: POP      ECX                                  
  0x363DAF: POP      ECX                                  
  ; ev.c - Line 189
  ;     if (Estat.on) {
  0x363DB0: CMP      DWORD PTR [0x10014268],0x0           ; <==0x00363
+DA2(*-0xE), 0x00363D93(*-0x1D), 0x00363D84(*-0x2C)
  0x363DB7: JZ       0x363E2C                             ; (*+0x75) 
  ; ev.c - Line 190
  ;     if (fp->stats) {
  0x363DB9: MOV      EAX,DWORD PTR [EBP+0x8]              ; ARG:fp <pe
+_cbframe *>
  0x363DBC: CMP      DWORD PTR [EAX+0x8],0x0              
  0x363DC0: JZ       0x363DDA                             ; (*+0x1A) 
  ; ev.c - Line 191
  ;         Estat.scrub(fp->stats, wa);
  0x363DC2: PUSH     DWORD PTR [EBP-0x8]                  ; VAR:wa <pe
+_watcher *>
  0x363DC5: MOV      EAX,DWORD PTR [EBP+0x8]              ; ARG:fp <pe
+_cbframe *>
  0x363DC8: PUSH     DWORD PTR [EAX+0x8]                  
  0x363DCB: CALL     DWORD PTR [0x1001427C]               
  0x363DD1: POP      ECX                                  
  0x363DD2: POP      ECX                                  
  ; ev.c - Line 192
  ;         fp->stats = 0;
  0x363DD3: MOV      EAX,DWORD PTR [EBP+0x8]              ; ARG:fp <pe
+_cbframe *>
  0x363DD6: AND      DWORD PTR [EAX+0x8],0x0              
  ; ev.c - Line 194
  ;     }
  ;     if (CurCBFrame >= 0) {
  0x363DDA: CMP      DWORD PTR [0x10012020],0x0           ; <==0x00363
+DC0(*-0x1A)
  0x363DE1: JL       0x363E2C                             ; (*+0x4B) 
  ; ev.c - Line 195
  ;         pe_cbframe *pfp = CBFrame + CurCBFrame;
  0x363DE3: MOV      EAX,DWORD PTR [0x10012020]           
  0x363DE8: IMUL     EAX,EAX,0xC                          
  0x363DEB: ADD      EAX,0x10014438                       
  0x363DF0: MOV      DWORD PTR [EBP-0xC],EAX              ; VAR:pfp <p
+e_cbframe *>
  ; ev.c - Line 196
  ;         if (!pfp->stats)
  0x363DF3: MOV      EAX,DWORD PTR [EBP-0xC]              ; VAR:pfp <p
+e_cbframe *>
  0x363DF6: CMP      DWORD PTR [EAX+0x8],0x0              
  0x363DFA: JNZ      0x363E1F                             ; (*+0x25) 
  ; ev.c - Line 197
  ;         pfp->stats = Estat.enter(CurCBFrame, pfp->ev->up->max_cb_t
+m);
  0x363DFC: MOV      EAX,DWORD PTR [EBP-0xC]              ; VAR:pfp <p
+e_cbframe *>
  0x363DFF: MOV      EAX,DWORD PTR [EAX]                  
  0x363E01: MOV      EAX,DWORD PTR [EAX+0x8]              
  0x363E04: MOVSX    EAX,DWORD PTR [EAX+0x48]             
  0x363E08: PUSH     EAX                                  
  0x363E09: PUSH     DWORD PTR [0x10012020]               
  0x363E0F: CALL     DWORD PTR [0x1001426C]               
  0x363E15: POP      ECX                                  
  0x363E16: POP      ECX                                  
  0x363E17: MOV      ECX,DWORD PTR [EBP-0xC]              ; VAR:pfp <p
+e_cbframe *>
  0x363E1A: MOV      DWORD PTR [ECX+0x8],EAX              
  ; ev.c - Line 198
  ;         else
  0x363E1D: JMP      0x363E2C                             ; (*+0xF) 
  ; ev.c - Line 199
  ;         Estat.resume(pfp->stats);
  0x363E1F: MOV      EAX,DWORD PTR [EBP-0xC]              ; VAR:pfp <p
+e_cbframe *>; <==0x00363DFA(*-0x25)
  0x363E22: PUSH     DWORD PTR [EAX+0x8]                  
  0x363E25: CALL     DWORD PTR [0x10014274]               
  0x363E2B: POP      ECX                                  
  ; ev.c - Line 203
  ;     }
  ;     }
  ;     /* this must be last because it can destroy the watcher */
  ;     pe_event_release(ev);
  0x363E2C: PUSH     DWORD PTR [EBP-0x4]                  ; VAR:ev <pe
+_event *>; <==0x00363DE1(*-0x4B), 0x00363E1D(*-0xF), 0x00363DB7(*-0x7
+5)
  0x363E2F: CALL     0x363E37                             ; SYM:pe_eve
+nt_release <Void>
  0x363E34: POP      ECX                                  
  ; ev.c - Line 204
  ; }
  0x363E35: LEAVE                                         
  0x363E36: RET                                           
  ;*******************************************************************
+*************
[download]

And a stack trace:

PID: 4020 TID: 4192 - Stack Contents for 0x00363D7C
0x00363D7C: Event.dll:pe_event_postCB + 0x0025 <Void>
0x00363C6F: Event.dll:pe_event_invoke + 0x0738 <Void>
0x00364EA1: Event.dll:pe_empty_queue + 0x0031 <Int>
0x00364ACC: Event.dll:one_event + 0x00ED <Int>
0x280933F9: Perl_get_context + 0x001F
0x00365A0E: Event.dll:XS_Event__loop + 0x011B <Void>
0x2804002B: Perl_sv_compile_2op + 0x7B9F
0x2805D4AF: Perl_runops_standard + 0x000C
0x28088A87: RunPerl + 0x0086
0x77F53CB3: RtlDosPathNameToNtPathName_U + 0x02E3
0x00401012: 
0x77E814C7: GetCurrentDirectoryW + 0x0044
[download]

I'm intrigued by what you are trying to achieve by mixing these two anyway?