The code to execute BEGIN immediately doesn't properly check the compilation error flag before executing the block. Here's a patch against 5.8.3:

--- op.c.orig   2004-03-22 16:59:09.000000000 -0800
+++ op.c        2004-03-22 16:59:25.000000000 -0800
@@ -4359,7 +4359,7 @@
        if (*s != 'B' && *s != 'E' && *s != 'C' && *s != 'I')
            goto done;
  
-       if (strEQ(s, "BEGIN")) {
+       if (strEQ(s, "BEGIN") && !PL_error_count) {
            I32 oldscope = PL_scopestack_ix;
            ENTER;
            SAVECOPFILE(&PL_compiling);
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It already appears to be fixed in 5.9.1.