I dont see or understand how this last example demonstrate that $\ cant be used this way. I see all special variables working in the very same way as expected: how many we have seen foreach $a... dont use $a pattern?

All special vars have the same behaviour:

use strict;
use warnings;

while (<DATA>){
    my $var = (split)[0];
    {    
        my $eval = "for $var ('OK '){ print $var }";
        local $@;
        eval $eval;
        print "$eval\n";
        if ( $@ ){
            print "\nEVAL ERROR: $@\n";
        }
    }
}

# data (with some modifications) from https://www.perlmonks.org/?node_
+id=353259
__DATA__
$NOT_AVAILABLE   #  A NON EXISTING SPECIAL VARIABLE
$_    The default or implicit variable.
$a    sort comparison routine variables.
$b    sort comparison routine variables.
$1    Regexp parenthetical capture holders.
$2    Regexp parenthetical capture holders.
$31  # JUST TO EXAGERATE
$&    Last successful match (degrades performance).
${^MATCH}    Similar to $& without performance penalty. Requires /p mo
+difier.
$`    Prematch for last successful match string (degrades performance)
+.
${^PREMATCH}    Similar to $` without performance penalty. Requires /p
+ modifier.
$'    Postmatch for last successful match string (degrades performance
+).
${^POSTMATCH}    Similar to $' without performance penalty. Requires /
+p modifier.
$+    Last paren match.
$^N    Last closed paren match (last submatch).
$^R    Last regexp (?{code}) result.
${^RE_DEBUG_FLAGS}    Current value of regexp debugging flags. See use
+ re 'debug';
${^RE_TRIE_MAXBUF}    Control memory allocations for RE optimizations 
+for large alternations.
${^ENCODING}    The object reference to the Encode object, used to con
+vert the source code to Unicode.
${^OPEN}    Internal use: \0 separated Input / Output layer informatio
+n.
${^UNICODE}    Read-only Unicode settings.
${^UTF8CACHE}    State of the internal UTF-8 offset caching code.
${^UTF8LOCALE}    Indicates whether UTF8 locale was detected at startu
+p.
$.    Current line number (or record number) of most recent filehandle
+.
$/    Input record separator.
$|    Output autoflush. 1=autoflush, 0=default. Applies to currently s
+elected handle.
$,    Output field separator (lists)
$\    Output record separator.
$"    Output list separator. (interpolated lists)
$;    Subscript separator. (Use a real multidimensional array instead.
+)
$%    Page number for currently selected output channel.
$=    Current page length.
$-    Number of lines left on page.
$~    Format name.
$^    Name of top-of-page format.
$:    Format line break characters
$^L    Form feed (default "\f").
$^A    Format Accumulator
$?    Child error. Status code of most recent system call or pipe.
$!    Operating System Error. (What just went 'bang'?)
$^E    Extended Operating System Error (Extra error explanation).
${^CHILD_ERROR_NATIVE}    Native status returned by the last pipe clos
+e, backtick (`` ) command, successful call to wait() or waitpid(), or
+ from the system() operator.
$$    Process ID
$<    Real user id of process.
$>    Effective user id of process.
$(    Real group id of process.
$)    Effective group id of process.
$0    Program name.
$^O    Operating System name.
$]    Old: Version and patch number of perl interpreter. Deprecated.
$^C    Current value of flag associated with -c switch.
$^D    Current value of debugging flags
$^F    Maximum system file descriptor.
$^I    Value of the -i (inplace edit) switch.
$^M    Emergency Memory pool.
$^P    Internal variable for debugging support.
$^R    Last regexp (?{code}) result.
$^S    Exceptions being caught. (eval)
$^T    Base time of program start.
$^V    Perl version.
$^W    Status of -w switch
${^WARNING_BITS}    Current set of warning checks enabled by use warni
+ngs;
$^X    Perl executable name.
${^GLOBAL_PHASE}    Current phase of the Perl interpreter.
$^H    Internal use only: Hook into Lexical Scoping.
${^TAINT}    Taint mode read-only flag.
${^WIN32_SLOPPY_STAT}    If true on Windows stat() won't try to open t
+he file.
[download]

My personal opinion is that this behaviour is correct and no errors nor warnings should be raised: such vars are always present and are somthing like super globals (is for such reason we localize $@ if needed) and if someone doesnt know about special vars and is so fool to use them this way is not a perl problem.

I like special variables and I will even add more to the list (to obtain switches passed to perl like -l )

L*