you will get a warning if you don't have the while (defined (my $line =<DATA>)){} syntax, that is because a line with just "0" could evaluate to "false". On "real file handles" the action is a bit different

Sorry, but no, DATA is a real filehandle, there shouldn't be any differences. And in regards to this "Perl has changed how it deals with <DATA>", I can't verify that, the defined check seems to be implicit all the way back to Perl 5.6 (I haven't had a chance to look at older versions). And I haven't been able to produce a warning either, or get while (<DATA>) or while (my $line=<DATA>) to evaluate a line containing only 0 (no newline) as false, on 5.6 thru 5.28. Interestingly, the implicit defined check wasn't actually documented in detail until recently (5e979393c).

I did figure this out at Re^3: First attempt at bringing in file for input/output.

This error happens when a conditional is used in the while() statement.

... with strict; and warnings; active, when reading from the predefined DATA file handle, you will get a warning if you don't have the while (defined (my $line =<DATA>)){} syntax ... a line with just "0" could evaluate to "false".

I'm confused about what you intended to say in the quoted text. A
    while (<DATA>) { ... }
is always implicitly
    while (defined($_ = <DATA>)) { ... }
or more fundamentally
    while (defined($_ = readline(DATA))) { ... }
for  DATA or any other filehandle. Even if a  '0' with no newline is the last character in a file or  __DATA__ or  __END__ block, no warning will be printed. (IIRC, this became true in a very early, but still non-zero, sub-version of Perl 5 — or was it true even for 5.0.0?) (Update: The following code was tested under ActiveState Perl version 5.8.9.)

Another way to see this is by deparsing:

c:\@Work\Perl\monks\Marshall>perl -MO=Deparse,-p t_read_DATA_1.pl
use warnings;
use strict 'refs';
while (defined(($_ = <DATA>))) {
    print($_);
}
__DATA__
line the first
second line
penultimate line is line 3
t_read_DATA_1.pl syntax OK
0
[download]

Update: I really should have mentioned that the preceding code was run under ActiveState Perl version 5.8.9, the ancientest Perl I hold in captivity.

I tested this with my Perl

C:\Users\mmtho\Documents\PerlProjects>perl -v

This is perl 5, version 24, subversion 3 (v5.24.3) built for MSWin32-x
+64-multi-thread
(with 1 registered patch, see perl -V for more detail)

...
[download]

And I found out that Perl has changed how it deals with <DATA>.
This works with Perl 5.24:

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

while (my $line = <DATA>)
{
   print $line;
}


__DATA__
aBC 
XYAS
[download]

In an oversight since corrected, I neglected to mention that the code here was run under Perl version 5.8.9.

I tried

while (my $line = <DATA>)
{
   print $line;
}
[download]

with my original  __DATA__ both with and without a newline after the 0 on the final line and with strictures and warnings fully enabled and under 5.8.9 and saw no difference in behavior (and no warnings). The reason is easy to see from a deparse:

c:\@Work\Perl\monks\Marshall>perl -MO=Deparse,-p t_read_DATA_2.pl
use warnings;
use strict 'refs';
print("perl version: $] \n\n");
while (defined((my $line = <DATA>))) {
    do {
        print($line)
    };
}
__DATA__
line the first
second line
penultimate line is line 3
t_read_DATA_2.pl syntax OK
0
[download]

The defined test is automatically added.

---

Give a man a fish:  <%-{-{-{-<