... 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.)

# t_read_DATA_1.pl                                          12oct18waw

use warnings;
use strict;

while (<DATA>) {
    print;
    }

__DATA__
line the first
second line
penultimate line is line 3
0
[download]

c:\@Work\Perl\monks\Marshall>od -t x1 t_read_DATA_1.pl | tail
0000120 72 6e 69 6e 67 73 3b 0d 0a 75 73 65 20 73 74 72
0000140 69 63 74 3b 0d 0a 0d 0a 77 68 69 6c 65 20 28 3c
0000160 44 41 54 41 3e 29 20 7b 0d 0a 20 20 20 20 70 72
0000200 69 6e 74 3b 0d 0a 20 20 20 20 7d 0d 0a 0d 0a 5f
0000220 5f 44 41 54 41 5f 5f 0d 0a 6c 69 6e 65 20 74 68
0000240 65 20 66 69 72 73 74 0d 0a 73 65 63 6f 6e 64 20
0000260 6c 69 6e 65 0d 0a 70 65 6e 75 6c 74 69 6d 61 74
0000300 65 20 6c 69 6e 65 20 69 73 20 6c 69 6e 65 20 33
0000320 0d 0a 30
0000323

c:\@Work\Perl\monks\Marshall>perl t_read_DATA_1.pl
line the first
second line
penultimate line is line 3
0
[download]

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.