in reply to how do I "initialize" $_

The default scalar  $_ (see perlvar) is assigned to in some cases, but not all; you've chosen one of the "not all" cases. A while-loop like 

while (<FILEHANDLE>) {
    print;
    }

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is really 
while (defined($_ = <FILEHANDLE>)) {
    print $_;
    }

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and likewise 
for (<FILEHANDLE>) {
    print;
    }

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is 
for $_ (<FILEHANDLE>) {
    print $_;
    }

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(Note that the while-loop will read and print the filehandle line-by-line, but the for-loop will read all lines of the file at once (could be a lotta lines!) and then print the lines one-by-one.) See perlsyn.

Give a man a fish:  <%-{-{-{-<

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Re^2: how do I "initialize" $_
by Anonymous Monk on Oct 11, 2018 at 06:46 UTC
    Thanks. I knew other ways of reading the data (assigning it a variable or using a loop), but I didn't know why the "automatic" assignment didn't work in this case, and you explained that the best.
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      Yes, BTW, your code: <FILEHANDLE>; just causes the first line to be discarded since it is not assigned to anything.

      I guess as another point, 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, that is because a line with just "0" could evaluate to "false". On "real file handles" the action is a bit different - Perl is very good at "doing what I meant" and the defined() is implicit. Perl is a huge language and abounds with little fine differences.

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        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).

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        ... 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.)

        Read more... (1329 Bytes)

        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

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        See O and B::Deparse. (I think the final  "0" appears in console output before the  "syntax OK" message because the  "0" is not terminated by a newline and doesn't get flushed until the process ends. The  "syntax" message may also be going to STDERR. Or something like that...)

        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.

        Give a man a fish:  <%-{-{-{-<

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