#!/usr/bin/perl -w
$_= "193287461528";
"0 but true" while s/(\d)(\d\d\d)(\D|$)/$1,$2$3/;
print;
produces:
Useless use of a constant in void context at warn.pl line 3.
193,287,461,528
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;
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
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;
-
tye
(but my friends call me "Tye") | 