It happens, the important thing is that you updated your post :-)

Interesting related factoid: Perl treats the string "0 but true" specially, it is exempt from the warnings about numeric conversion, so it is also true in boolean context but zero in numeric context. (see e.g. ioctl and fcntl)

Yes, there is "0 but true" and that is very, very seldom known thing. You are correct about that extra detail that I'd forgotten, but I have seen it before.

Thanks for adding that point!

My main point got lost in that there is such a thing as "True but Zero value". I just blew it in my post. With my current small screen, number "0" and letter "O" look almost the same. I blew it. I admit it. Let's move on.

Update: Here is some code that proves your comment:

#!/usr/bin/perl
use warnings;
use strict;

my $x = "0 but true";
my $y = "0 but whatever";

$x += 0;
$y += 0;
__END__

Prints:
Argument "0 but whatever" isn't numeric in 
addition (+) at C:\Projects_Perl\0buttrue.pl line 9.

Process completed successfully
[download]

You are correct, my $x = "0 but true"; is "special". 0E0 is more often seen, but this works the same way.

Hello Marshall,

0E0 is more often seen, but this works the same way.

Yes, but only when it appears as a string; when it appears as a number, it is zero and false:

#! perl
use strict;
use warnings;

for (
        0,
        0E0,
       '0E0',
        0.0,
       '0.0',
        0.00,
       '0.00',
        00,
       '00',
       '0',
       '0 but true',
       '0 BUT TRUE',
       '0 but true ',
       '1 and true',
    )
{
    printf "%-11s  %d  %s\n", $_, ($_ + 0), ($_ ? 'true' : 'false');
}
[download]

Output:

13:25 >perl 1575_SoPW.pl
0            0  false
0            0  false
0E0          0  true
0            0  false
0.0          0  true
0            0  false
0.00         0  true
0            0  false
00           0  true
0            0  false
0 but true   0  true
Argument "0 BUT TRUE" isn't numeric in addition (+) at 1575_SoPW.pl li
+ne 34.
0 BUT TRUE   0  true
Argument "0 but true " isn't numeric in addition (+) at 1575_SoPW.pl l
+ine 34.
0 but true   0  true
Argument "1 and true" isn't numeric in addition (+) at 1575_SoPW.pl li
+ne 34.
1 and true   1  true

13:25 >
[download]

The string '0' is documented as special: it is zero and false. Other strings are true¹ regardless of their numerical values. It appears that the isn't numeric warning applies only when the numerical conversion is not “pure” (because the string contains non-numeric characters).

It seems strange that this behaviour is so poorly documented. The '0 but true' special case is not mentioned in perlsyn#Truth-and-Falsehood. My experiments suggest that this special string must be character-for-character exact for the special behaviour to apply. For example, '0 but true ' (note the extra space at the end) still generates an isn't numeric warning.

Update: ¹Except the empty string, '', which is also documented to be false.

Hope that helps,

Athanasius <°(((>< contra mundum

Iustus alius egestas vitae, eros Piratica,