/* from op.c */

if (ckWARN(WARN_VOID)) {
  useless = "a constant";  
  /* the constants 0 and 1 are permitted as they are
     conventionally used as dummies in constructs like
     1 while some_condition_with_side_effects;  */  
  if (SvNIOK(sv) && (SvNV(sv) == 0.0 || SvNV(sv) == 1.0))
    useless = 0;
  else if (SvPOK(sv)) {
    /* perl4's way of mixing documentation and code
       (before the invention of POD) was based on a
       trick to mix nroff and perl code. The trick was
       built upon these three nroff macros being used in
       void context. The pink camel has the details in   
       the script wrapman near page 319. */
    if (strnEQ(SvPVX(sv), "di", 2) ||
        strnEQ(SvPVX(sv), "ds", 2) ||
        strnEQ(SvPVX(sv), "ig", 2))
          useless = 0;
  }
}
[download]

#!/usr/bin/perl -w
$_= "193287461528";
"0 but true" while s/(\d)(\d\d\d)(\D|$)/$1,$2$3/;
print;
[download]

produces:

Useless use of a constant in void context at warn.pl line 3.
193,287,461,528
[download]

because the "0 but true" doesn't yet have a numeric value so the "is 0 or 1" test doesn't even get attempted.

To get "0 but true" to work as an "unwarned useless constant", you have to jump through some interesting hoops:

#!/usr/bin/perl -w
$_= "193287461528";
sub zero() { "0 but true" }
BEGIN {
    if(  @ARGV  ) {
        eval '0+zero';
    }
}
zero while s/(\d)(\d\d\d)(\D|$)/$1,$2$3/;
print;
[download]

produces

$ perl warn.pl
Useless use of a constant in void context at warn.pl line 9.
193,287,461,528
$ perl warn.pl x
193,287,461,528
[download]

The "()" in "sub zero()" is required for Perl to compile uses of "zero" as a constant (instead of as a subroutine call). The "0+zero" causes Perl to cache a numeric value for the "zero" constant. But you'll note that this caching is done at compile time so I have to use stringy eval to prevent it from happening when I don't want it to. But stringy eval also prevents it from happening at compile time even if that code gets run so I have to use a BEGIN block to get conditional compile-time behavior.

So, how to use "0 but true" as a unwarned useless constant can be reduced to:

#!/usr/bin/perl -w
$_= "193287461528";
sub zero() { "0 but true" }
zero while s/(\d)(\d\d\d)(\D|$)/$1,$2$3/;
print;
my $x= 0+zero;
[download]

        - tye (but my friends call me "Tye")