#!/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")