use constant usage clarification

[perlron]

Hello Monks
I am not able to understand the following line of code.
use constant x => 1/3 in the context that the following line of code gives a scalar assignment error
use constant x = 1/3
From what i have read in perldoc -f use, the former is a pragma. I tried to check in the perlpragma for this usage reference, but i could only such case where the => was mentioned , without a clear reference as to its usage.
The background is im writing a file to a PDF, and while reading a document on how to use PDF:API2 and i came across this code instance which seemed relevant to fully understand

[Loops]

Hi perlron,

from the perlop page:

The => operator is a synonym for the comma except that it causes a word on its left to be interpreted as a string if it begins with a letter or underscore and is composed only of letters, digits and underscores. This includes operands that might otherwise be interpreted as operators, constants, single number v-strings or function calls. If in doubt about this behavior, the left operand can be quoted explicitly.

So there is no assignment going on (at that point), just a list of values being passed to the pragma. It's the same as this:

use constant 'x',  1/3;

The pragma then does the magic of turning each identifier,value pair into a usable constant.

[perlron]

Ok .thanks . nothing to worry for me about => then..

[Anonymous Monk]

I am not able to understand the following line of code.

Its right there in Basic debugging checklist , B::Deparse

$ perl -MO=Deparse,-p -e " use constant X => 1/3; "
use constant ('X', 0.33333333333333331);
-e syntax OK

When you're not sure about perl syntax, get App::PPI::Dumper

ppi_dumper / wxPPIxregexplain.pl

$ cat 2
use constant X => 1/3;


$ ppi_dumper 2
PPI::Document
  PPI::Statement::Include
    PPI::Token::Word    'use'
    PPI::Token::Whitespace      ' '
    PPI::Token::Word    'constant'
    PPI::Token::Whitespace      ' '
    PPI::Token::Word    'X'
    PPI::Token::Whitespace      ' '
    PPI::Token::Operator        '=>'
    PPI::Token::Whitespace      ' '
    PPI::Token::Number          '1'
    PPI::Token::Operator        '/'
    PPI::Token::Number          '3'
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Token::Whitespace        '\n'

So what is  =>? Its an PPI::Token::Operator ... see perlop

[BrowserUk]

[this]' gives a scalar assignment error use constant x = 1/3

Really? Cos when I try it, the error is: Unquoted string "x" may clash with future reserved word.

Care to post a short script that produces that error when run?

[Discipulus]

Hello BrowserUK i'm receiving too an assignement error using the wrong syntax with the not stringified left value:


#(something like:)
perl -MConfig -e "BEGIN{print 'Perl ', map {$Config{$_},' '}qw/version
+ os archname/ }; use constant X = 1/3;print qq(\n),+X,qq(\n)"

  ----------------------------------------·······
 | Can't modify constant item in scalar assignment at -e line 1, near 
+"3;"
 |  BEGIN not safe after errors--compilation aborted at -e line 1.
 |  Perl 5.14.2  MSWin32-x86-multi-thread
  --------------------------------------------------------------------
+---·······

  ----------------------------------------·······
 | Can't modify constant item in scalar assignment at -e line 1, near 
+"3;"
 |  BEGIN not safe after errors--compilation aborted at -e line 1.
 |  Perl 5.20.0  MSWin32-x86-multi-thread-64int
  --------------------------------------------------------------------
+---·······

  ----------------------------------------·······
 | Can't modify constant item in scalar assignment at -e line 1, near 
+"3;"
 |  BEGIN not safe after errors--compilation aborted at -e line 1.
 |  Perl 5.16.2  MSWin32-x64-multi-thread
  --------------------------------------------------------------------
+---·······


L*