Why don't I get output when I use __DATA__ in Perl? - perl

Question

Does anybody know if the Perl __DATA__ syntax on macOS Catalina is deprecated? I have perl v5.18.4 running, even a simple program like this gives no output (and no error either);
use strict;
use warnings;
while(<DATA>){
print $_;
}
__DATA__
line1
line2
line3
Edit:
This is weird. I said earlier that I have 2 Mac systems, both having the same problem. Not quite right, on one system the program works, on the other system the same program doesn’t.
Hexdump on both systems is the same:
Mac Mini:
Mac-mini-van-Theo:Programming theo$ hexdump -C test.pl
00000000 75 73 65 20 73 74 72 69 63 74 3b 0d 75 73 65 20 |use strict;.use |
00000010 77 61 72 6e 69 6e 67 73 3b 0d 77 68 69 6c 65 28 |warnings;.while(|
00000020 3c 44 41 54 41 3e 29 20 7b 0d 20 20 20 20 70 72 |<DATA>) {. pr|
00000030 69 6e 74 20 24 5f 3b 0d 7d 0d 5f 5f 44 41 54 41 |int $_;.}.__DATA|
00000040 5f 5f 0d 6c 69 6e 65 31 0d 6c 69 6e 65 32 0d 6c |__.line1.line2.l|
00000050 69 6e 65 33 0d |ine3.|
00000055
iMac:
Theo#iMac-van-Theo Programming % hexdump -C test.pl
00000000 75 73 65 20 73 74 72 69 63 74 3b 0a 75 73 65 20 |use strict;.use |
00000010 77 61 72 6e 69 6e 67 73 3b 0a 77 68 69 6c 65 28 |warnings;.while(|
00000020 3c 44 41 54 41 3e 29 7b 0a 20 20 20 20 70 72 69 |<DATA>){. pri|
00000030 6e 74 20 24 5f 3b 0a 7d 0a 0a 5f 5f 44 41 54 41 |nt $_;.}..__DATA|
00000040 5f 5f 0a 6c 69 6e 65 31 0a 6c 69 6e 65 32 0a 6c |__.line1.line2.l|
00000050 69 6e 65 33 0a |ine3.|
00000055
However, a 'cat’ or a ‘more’ shows differences:
Mac Mini:
Mac-mini-van-Theo:Programming theo$ more test.pl
use strict;^Muse warnings;^Mwhile(<DATA>) {^M print $_;^M}^M__DATA__^Mline1^Mline2^Mline3
iMac:
Theo#iMac-van-Theo Programming % more test.pl
use strict;
use warnings;
while(<DATA>){
print $_;
}
__DATA__
line1
line2
line3
The difference? The Mac Mini uses ‘bash’ as shell (where the program fails), the iMac uses ‘zsh’. So the problem is not really perl related but perl/shell related. With Catalina, Zsh is used as the default shell but the old Bash shell is still included with macOS and you can still switch to it. It seems to be related to how the shell handles line-endings, although I do not understand why this happens and moreover how to solve it.

Answer 1

__DATA__ is just fine, and there's no platform-specific issues with it (and there are lots of stupid tricks you can do with it).
However, if you want to know the state of any particular Perl thing, there's the perldeprecation docs. Sometimes perlexperiment is handy too.
How are you running your program?
Supply a hexdump of your program: hexdump -C program.pl. Maybe there are funny characters.