What do the non-printable characters in the Perl symbol table represent? - perl

Question

I just learned that in Perl, the symbol table for a given module is stored in a hash that matches the module name -- so, for example, the symbol table for the fictional module Foo::Bar would be %Foo::Bar. The default symbol table is stored in %main::. Just for the sake of curiosity, I decided that I wanted to see what was in %main::, so iterated through each key/value pair in the hash, printing them out as I went:
#! /usr/bin/perl
use v5.14;
use strict;
use warnings;
my $foo;
my $bar;
my %hash;
while( my ( $key, $value ) = each %:: ) {
say "Key: '$key' Value '$value'";
}
The output looked like this:
Key: 'version::' Value '*main::version::'
Key: '/' Value '*main::/'
Key: '' Value '*main::'
Key: 'stderr' Value '*main::stderr'
Key: '_<perl.c' Value '*main::_<perl.c'
Key: ',' Value '*main::,'
Key: '2' Value '*main::2'
...
I was expecting to see the STDOUT and STDERR file handles, and perhaps #INC and %ENV... what I wasn't expecting to see was non-ascii characters ... what the code block above doesn't show is that the third line of the output actually had a glyph indicating a non-printable character.
I ran the script and piped it as follows:
perl /tmp/asdf.pl | grep '[^[:print:]]' | while read line
do
echo $line
od -c <<< $line
echo
done
The output looked like this:
Key: '' Value '*main::'
0000000 K e y : ' 026 ' V a l u e '
0000020 * m a i n : : 026 ' \n
0000032
Key: 'ARNING_BITS' Value '*main::ARNING_BITS'
0000000 K e y : ' 027 A R N I N G _ B I
0000020 T S ' V a l u e ' * m a i n
0000040 : : 027 A R N I N G _ B I T S ' \n
0000060
Key: '' Value '*main::'
0000000 K e y : ' 022 ' V a l u e '
0000020 * m a i n : : 022 ' \n
0000032
Key: 'E_TRIE_MAXBUF' Value '*main::E_TRIE_MAXBUF'
0000000 K e y : ' 022 E _ T R I E _ M A
0000020 X B U F ' V a l u e ' * m a
0000040 i n : : 022 E _ T R I E _ M A X B
0000060 U F ' \n
0000064
Key: ' Value '*main:'
0000000 K e y : ' \b ' V a l u e '
0000020 * m a i n : : \b ' \n
0000032
Key: '' Value '*main::'
0000000 K e y : ' 030 ' V a l u e '
0000020 * m a i n : : 030 ' \n
0000032
So what are non-printable characters doing in the Perl symbol table? What are they symbols for?

Answer 1

Guru is on the right track: specifically, the answer is to be found in perlvar, which says:
"Perl variable names may also be a sequence of digits or a single punctuation or control character. These names are all reserved for special uses by Perl; for example, the all-digits names are used to hold data captured by backreferences after a regular expression match. Perl has a special syntax for the single-control-character names: It understands ^X (caret X) to mean the control-X character. For example, the notation $^W (dollar-sign caret W) is the scalar variable whose name is the single character control-W. This is better than typing a literal control-W into your program.
Since Perl 5.6, Perl variable names may be alphanumeric strings that begin with control characters (or better yet, a caret). These variables must be written in the form ${^Foo}; the braces are not optional. ${^Foo} denotes the scalar variable whose name is a control-F followed by two o's. These variables are reserved for future special uses by Perl, except for the ones that begin with ^_ (control-underscore or caret-underscore). No control-character name that begins with ^_ will acquire a special meaning in any future version of Perl; such names may therefore be used safely in programs. $^_ itself, however, is reserved."
If you want to print those names in a readable way, you could add a line like this to your code:
$key = '^' . ($key ^ '#') if $key =~ /^[\0-\x1f]/;
If first character of $key is a control character, this will replace it with a caret followed by the corresponding letter (^A for control-A, ^B for control-B, etc.).

Answer 2

Perl has special variables such as $", $, , $/ , $\ and so on. All these are part of symbol table which is what you are seeing. Also, you should be able to see #INC, %ENV in the symbol table as well.