From mjtg@cus.cam.ac.uk

perldata says​:

  Within search patterns (which also undergo double-quotish
  substitution) there is a bad ambiguity​: Is /$foo[bar]/ to
  be interpreted as /${foo}[bar]/ (where [bar] is a character
  class for the regular expression) or as /${foo[bar]}/ (where <<<<<<
  [bar] is the subscript to array @​foo)? If @​foo doesn't
  otherwise exist, then it's obviously a character class. If
  @​foo exists, Perl takes a good guess about [bar], and is
  almost always right. If it does guess wrong, or if you're
  just plain paranoid, you can force the correct
  interpretation with curly brackets as above.

which I interpret as meaning it should work for substitutions in
search patterns at least. And experiment shows that it works in most
other cases. So this prints 213312 as expected​:

  @​a = (2,3,4);
  $h{${a[0]}} = 13;
  ${h{$a[1]}} = 12;
  print %h;

But if you try to nest them, there is a syntax error (and parsing stops
immediately rather than continuing to the end of the script)​:

  @​a = (2,3,4);
  ${h{${a[0]}}} = 13;
  This line isn't parsed.

gives

  syntax error at - line 2, near "}}"
  Execution of - aborted due to compilation errors.

Mike Guy

% perl -V
Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration​:
  Platform​:
  osname=sunos, osvers=4.1.3, archname=sun4-sunos
  uname=''
  hint=previous, useposix=true, d_sigaction=define
  usethreads=undef useperlio=undef d_sfio=undef
  Compiler​:
  cc='gcc', optimize='-O', gccversion=2.7.2.3
  cppflags='-I/usr/local/include -DREG_INFTY=22790'
  ccflags ='-I/usr/local/include -DREG_INFTY=22790'
  stdchar='unsigned char', d_stdstdio=define, usevfork=true
  intsize=4, longsize=4, ptrsize=4, doublesize=8
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=8
  alignbytes=8, usemymalloc=y, prototype=define
  Linker and Libraries​:
  ld='ld', ldflags =' -L/usr/local/lib'
  libpth=/usr/local/lib /lib /usr/lib /usr/ucblib
  libs=-ldbm -ldl -lm -lc -lposix
  libc=/lib/libc.so.1.8, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
  cccdlflags='-fpic', lddlflags='-assert nodefinitions -L/usr/local/lib'

Characteristics of this binary (from libperl)​:
  Locally applied patches​:
  pos.list
  IO.file.return
  int.delimiter
  bigfloat
  Built under sunos
  Compiled at Jul 7 1999 16​:09​:54
  @​INC​:
  /home/mjtg/perl5.005_03/lib
  /home/mjtg/perl5.005_03/lib
  /home/mjtg/perl5.005_03/lib
  .

From @TimToady

I'm working on this one, in case anyone else was thinking about it.
Basically, lex_fakebrack needs to go away.

Larry

From [Unknown Contact. See original ticket]

Larry Wall writes​:

I'm working on this one, in case anyone else was thinking about it.
Basically, lex_fakebrack needs to go away.

Thanks. There are not so many *totally* incomprehensible places in
toke.c, and lex_fakebrack is one of them. Probably one could not
grok it because it was coded wrong on the first place... ;-)

Ilya

From @TimToady

M.J.T. Guy writes​:
: @​a = (2,3,4);
: ${h{${a[0]}}} = 13;
: This line isn't parsed.
:
: gives
:
: syntax error at - line 2, near "}}"
: Execution of - aborted due to compilation errors.

Here's the patch. Be sure to make regen_headers. We might be able to
do something similar with lex_formbrack, but I chickened out. Bock, bock.

Larry

diff -ru perl5.005_62/intrpvar.h perl5.005_62pat/intrpvar.h
--- perl5.005_62/intrpvar.h	Tue Oct  5 16:29:09 1999
+++ perl5.005_62pat/intrpvar.h	Tue Dec  7 10:14:18 1999
@@ -252,7 +252,6 @@
 PERLVAR(Ilex_expect,	expectation)	/* expect after determined token */
 PERLVAR(Ilex_brackets,	I32)		/* bracket count */
 PERLVAR(Ilex_formbrack,	I32)		/* bracket count at outer format level */
-PERLVAR(Ilex_fakebrack,	I32)		/* outer bracket is mere delimiter */
 PERLVAR(Ilex_casemods,	I32)		/* casemod count */
 PERLVAR(Ilex_dojoin,	I32)		/* doing an array interpolation */
 PERLVAR(Ilex_starts,	I32)		/* how many interps done on level */
diff -ru perl5.005_62/toke.c perl5.005_62pat/toke.c
--- perl5.005_62/toke.c	Mon Oct 11 22:22:11 1999
+++ perl5.005_62pat/toke.c	Tue Dec  7 11:11:12 1999
@@ -31,6 +31,9 @@
 static void restore_expect(pTHXo_ void *e);
 static void restore_lex_expect(pTHXo_ void *e);
 
+#define XFAKEBRACK 128
+#define XENUMMASK 127
+
 #define UTF (PL_hints & HINT_UTF8)
 /*
  * Note: we try to be careful never to call the isXXX_utf8() functions
@@ -360,7 +363,6 @@
 
     SAVEI32(PL_lex_dojoin);
     SAVEI32(PL_lex_brackets);
-    SAVEI32(PL_lex_fakebrack);
     SAVEI32(PL_lex_casemods);
     SAVEI32(PL_lex_starts);
     SAVEI32(PL_lex_state);
@@ -387,7 +389,6 @@
     PL_lex_defer = 0;
     PL_expect = XSTATE;
     PL_lex_brackets = 0;
-    PL_lex_fakebrack = 0;
     New(899, PL_lex_brackstack, 120, char);
     New(899, PL_lex_casestack, 12, char);
     SAVEFREEPV(PL_lex_brackstack);
@@ -963,7 +964,6 @@
     PL_lex_state = PL_sublex_info.super_state;
     SAVEI32(PL_lex_dojoin);
     SAVEI32(PL_lex_brackets);
-    SAVEI32(PL_lex_fakebrack);
     SAVEI32(PL_lex_casemods);
     SAVEI32(PL_lex_starts);
     SAVEI32(PL_lex_state);
@@ -988,7 +988,6 @@
 
     PL_lex_dojoin = FALSE;
     PL_lex_brackets = 0;
-    PL_lex_fakebrack = 0;
     New(899, PL_lex_brackstack, 120, char);
     New(899, PL_lex_casestack, 12, char);
     SAVEFREEPV(PL_lex_brackstack);
@@ -1036,7 +1035,6 @@
 	SAVEFREESV(PL_linestr);
 	PL_lex_dojoin = FALSE;
 	PL_lex_brackets = 0;
-	PL_lex_fakebrack = 0;
 	PL_lex_casemods = 0;
 	*PL_lex_casestack = '\0';
 	PL_lex_starts = 0;
@@ -2988,7 +2986,8 @@
 	    PL_lex_formbrack = 0;
 	if (PL_lex_state == LEX_INTERPNORMAL) {
 	    if (PL_lex_brackets == 0) {
-		if (PL_lex_fakebrack) {
+		if (PL_expect & XFAKEBRACK) {
+		    PL_expect &= XENUMMASK;
 		    PL_lex_state = LEX_INTERPEND;
 		    PL_bufptr = s;
 		    return yylex();	/* ignore fake brackets */
@@ -2999,9 +2998,9 @@
 		    PL_lex_state = LEX_INTERPEND;
 	    }
 	}
-	if (PL_lex_brackets < PL_lex_fakebrack) {
+	if (PL_expect & XFAKEBRACK) {
+	    PL_expect &= XENUMMASK;
 	    PL_bufptr = s;
-	    PL_lex_fakebrack = 0;
 	    return yylex();		/* ignore fake brackets */
 	}
 	force_next('}');
@@ -5609,8 +5608,6 @@
     char *bracket = 0;
     char funny = *s++;
 
-    if (PL_lex_brackets == 0)
-	PL_lex_fakebrack = 0;
     if (isSPACE(*s))
 	s = skipspace(s);
     d = dest;
@@ -5715,9 +5712,8 @@
 			"Ambiguous use of %c{%s%s} resolved to %c%s%s",
 			funny, dest, brack, funny, dest, brack);
 		}
-		PL_lex_fakebrack = PL_lex_brackets+1;
 		bracket++;
-		PL_lex_brackstack[PL_lex_brackets++] = XOPERATOR;
+		PL_lex_brackstack[PL_lex_brackets++] = XOPERATOR | XFAKEBRACK;
 		return s;
 	    }
 	}