Use C::Blocks to create a keyword hoook and investigate hints hash scoping at the ends of blocks.

keyword-scope.pl

use strict;
use warnings;
use C::Blocks;
use C::Blocks::PerlAPI;

# Write a little keyword hook that simply prints the keywords it encounters
clex {
	int (*next_keyword_plugin)(pTHX_ char *, STRLEN, OP **);
	int prev_count;
	
	int my_keyword_plugin(pTHX_ char *keyword_ptr, STRLEN keyword_len, OP **op_ptr) {
		printf("%s\t", keyword_ptr);
		
		PL_hints |= HINT_LOCALIZE_HH;
		gv_HVadd(PL_hintgv);
		
		// get the current value of the counter, with modifier flag set
		SV ** counter_p = hv_fetchs(GvHV(PL_hintgv), "counter", 1);
		if (!counter_p) {
			printf("hints-hash fetch failed!\n");
		}
		else {
			SV * counter = *counter_p;
			sv_inc(counter);
			int curr_count = SvIV(counter);
			if (curr_count != prev_count + 1) {
				printf(" *** just changed count -> ");
			}
			prev_count = curr_count;
			printf("%d potential keywords\n", curr_count);
		}
		
		return next_keyword_plugin(aTHX_ keyword_ptr, keyword_len, op_ptr);
	}
}

# Install the keyword hook
BEGIN {
	cblock {
		printf("Hooking parser\n");
		next_keyword_plugin = PL_keyword_plugin;
		PL_keyword_plugin = my_keyword_plugin;
		prev_count = 0;
	}
}

my $thing = 0;
for (1) {
	print "\$_ = $_\n";
	my $a;
	my $b;
	my $c;
}
;
printf "post-loop\n";
my $foo;

for (1) {
	print "\$_ = $_\n";
	my $a;
	my $b;
	my $c;
}
printf "post-loop (again)\n";