-p option with __DATA_

jdalbec has asked for the wisdom of the Perl Monks concerning the following question:

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Re: -p option with __DATA__ by jwkrahn (Abbot) on Nov 19, 2009 at 15:57 UTC
`#! /usr/bin/perl -p __DATA__ hello goodbye` [download] Is the same as: `#! /usr/bin/perl while ( <> ) { } continue { print } __DATA__ hello goodbye` [download] You are not doing anything with the `DATA` filehandle.	[reply] [d/l] [select]
Re^2: -p option with __DATA__ by toolic (Bishop) on Nov 19, 2009 at 16:10 UTC
And the OP can prove this using: `perl -MO=Deparse script.pl` [download] This is tip #6 from Basic debugging checklist.	[reply] [d/l]
Re^3: -p option with __DATA__ by jdalbec (Deacon) on Nov 21, 2009 at 01:12 UTC
I guess what confuses me is that `perl -MO=Deparse script.pl` [download] and `perl -MO=Deparse < script.pl` [download] give (almost) the same output, but `perl script.pl` [download] hangs as described, whereas `perl < script.pl` [download] prints what I expected.	[reply] [d/l] [select]
Re^4: -p option with __DATA__ by ikegami (Patriarch) on Nov 21, 2009 at 01:49 UTC
Re^2: -p option with __DATA__ by 7stud (Deacon) on Nov 19, 2009 at 16:17 UTC
...which uses the 'diamond operator'. Normally, the diamond operator takes all the file names specified on the command line as arguments, combines the corresponding files into one big file, and then assigns a line from the big file to $_ each time through the loop. However, if you do not specify any file names as arguments on the command line, then the diamond operator reads from STDIN. Presumably, that is what you did, so your program is waiting for you to type in a line of text for the diamond operator to process (you would hit ctrl+D to signal end-of-file after typing in all your lines). Also, when you specify filenames as arguments on the command line, if you specify a file name as: -, then the diamond operator will read from STDIN for that portion of the big file.	[reply]
Re^3: -p option with __DATA__ by ikegami (Patriarch) on Nov 19, 2009 at 17:09 UTC
Normally, the diamond operator takes all the file names specified on the command line as arguments, Only when `ARGV` is provided as the handle or not handle is provided (since the diamond operator uses `ARGV` by default). That's because the diamond operator does none of that. It's all `ARGV`'s doing. combines the corresponding files into one big file Yes, without actually combining the files on disk or in mem. In fact, using `eof` (with no argument or parens), you can detect when it's going to move to another file. and then assigns a line from the big file to $_ each time through the loop No, the diamond operator neither creates a loop nor assigns to `$_`. `while` provides the looping. However, if you do not specify any file names as arguments on the command line, then the diamond operator reads from STDIN. Again, only if ARGV is used as the handle since it's all `ARGV`'s doing. if you specify a file name as: -, then the diamond operator will read from STDIN for that portion of the big file. `ARGV` uses the two arg version of `open`. It also allows arguments such as `'zcat compressed.gz \|'`. In other words, without using a shell, you can achieve the same using `bash`'s `<( zcat compressed.gz )`	[reply] [d/l] [select]
Re: -p option with __DATA__ by ikegami (Patriarch) on Nov 19, 2009 at 16:03 UTC
`-p` is documented to do `while (<>)`, not `while (<DATA>)` The default file handle, `ARGV`, reads from the files named in `@ARGV` (i.e. supplied on the command line) or `STDIN` if no files were supplied.	[reply] [d/l] [select]
Re: -p option with __DATA__ by jethro (Monsignor) on Nov 19, 2009 at 16:04 UTC
The stuff in __DATA__ is read with the filehandle DATA, not with the magical default-filehandle of `while (<>) {`	[reply] [d/l]
Re: -p option with __DATA__ by codeacrobat (Chaplain) on Nov 19, 2009 at 19:44 UTC
try aliasing DATA to ARGV. `#!/usr/bin/perl -p BEGIN{ARGV=DATA} __DATA__ hello world` [download] `print+qq(\L@{[ref\&@]}@{['@'x7^'!#2/"!4']});`	[reply] [d/l] [select]