[perl #123893] Fix hang with "@{"

Commit v5.21.8-320-ge47d32d stopped code interpolated into quote-like
operators from reading more lines of input, by making lex_next_chunk
ignore the open filehandle and return false.  That causes this block
under case 0 in yylex to loop:

	    if (!lex_next_chunk(fake_eof)) {
		CopLINE_dec(PL_curcop);
		s = PL_bufptr;
		TOKEN(';');	/* not infinite loop because rsfp is NULL now */
	    }

(rsfp is not null there.)  This commit makes it check for quote-like
operators above, in the same place where it checks whether the file is
open, to avoid falling through to this code that can loop.

This changes the syntax errors for a couple of cases recently added
to t/op/lex.t, though I think the error output is now more consis-
tent overall.

(cherry picked from commit 0f9d53bbcafba2b30e50a1ad22c7759be170e14a)

2 files changed, 17 insertions, 5 deletions

diff --git a/t/op/lex.t b/t/op/lex.t
index ac094f8d5f..cbb72efaf2 100644
--- a/t/op/lex.t
+++ b/t/op/lex.t
@@ -4,7 +4,7 @@ use warnings;
 
 BEGIN { chdir 't'; require './test.pl'; }
 
-plan(tests => 10);
+plan(tests => 11);
 
 {
     no warnings 'deprecated';
@@ -91,15 +91,26 @@ is runperl(
 
 fresh_perl_is(
   '/$a[/<<a',
-  "syntax error at - line 1, next char ;\n" .
-  "Can't find string terminator \"a\" anywhere before EOF at - line 1.\n",
+  "Missing right curly or square bracket at - line 1, within pattern\n" .
+  "syntax error at - line 1, at EOF\n" .
+  "Execution of - aborted due to compilation errors.\n",
    { stderr => 1 },
   '/$a[/<<a with no newline [perl #123712]'
 );
 fresh_perl_is(
   '/$a[m||/<<a',
-  "syntax error at - line 1, next char ;\n" .
+  "Missing right curly or square bracket at - line 1, within pattern\n" .
+  "syntax error at - line 1, at EOF\n" .
   "Execution of - aborted due to compilation errors.\n",
    { stderr => 1 },
   '/$a[m||/<<a with no newline [perl #123712]'
 );
+
+fresh_perl_is(
+  '"@{"',
+  "Missing right curly or square bracket at - line 1, within string\n" .
+  "syntax error at - line 1, at EOF\n" .
+  "Execution of - aborted due to compilation errors.\n",
+   { stderr => 1 },
+  '"@{" [perl #123712]'
+);
diff --git a/toke.c b/toke.c
index f59f913486..50e2d59197 100644
--- a/toke.c
+++ b/toke.c
@@ -5198,7 +5198,8 @@ Perl_yylex(pTHX)
 	if (PL_madskills)
 	    PL_faketokens = 0;
 #endif
-	if (!PL_rsfp && (!PL_parser->filtered || s+1 < PL_bufend)) {
+	if ((!PL_rsfp || PL_lex_inwhat)
+	 && (!PL_parser->filtered || s+1 < PL_bufend)) {
 	    PL_last_uni = 0;
 	    PL_last_lop = 0;
 	    if (PL_lex_brackets &&