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The Trick

Frequently I find myself with a one-off data file that I need to analyze with a one-off script. It's nice to keep the script with the data, so I generally put the script in the data file, with a __DATA__ marker separating the two parts.

At this point, I have another problem. One-off scripts benefit tremendously by the use of Perl's command line flags that write code for you, the '-p' and '-n' flag in particular. Those two flags wrap your code in a while (<>) ... loop, which unfortunately reads all the files listed on the command line, or STDIN if there aren't any. My data, needless to say, is in the DATA file handle. I mentioned this in the chatterbox, and choroba had the answer:

BEGIN { *ARGV = *DATA unless @ARGV }

I especially like the unless clause, since it lets me override the data source. I can see using this for test cases, where I have a bunch of default test data but can easily test against other data files as well.

Why It Works

We're overwriting one typeglob (*ARGV) with another (*DATA). A typeglob contains Perl's internal representation of everything known about the given name, which includes any scalars, arrays, hashs, or filehandles. In this case, the ARGV set of variables have several "magical" properties, which are listed in perlvar:

Contains the name of the current file when reading from <>.
The array @ARGV contains the command-line arguments intended for the script. $#ARGV is generally the number of arguments minus one, because $ARGV[0] is the first argument, not the program's command name itself. See $0 for the command name.
The special filehandle that iterates over command-line filenames in @ARGV . Usually written as the null filehandle in the angle operator <> . Note that currently ARGV only has its magical effect within the <> operator; elsewhere it is just a plain filehandle corresponding to the last file opened by <> . In particular, passing \*ARGV as a parameter to a function that expects a filehandle may not cause your function to automatically read the contents of all the files in @ARGV.

The assignment *ARGV = *DATA will replace all of these with the only-slightly-less magical DATA values, which is cleverly not mentioned in perlvar, only in perldata. In this case, only the filehandle has any special properties. This means that the assignment also overwrites the $ARGV and @ARGV values with the undefined values of $DATA and @DATA, but I can't see many cases where you'd need those values once ARGV is gone. If I'm wrong, however, ambrus has pointed out that you could change the IO slot only, by *ARGV = *DATA{IO}

See Also...

'perl -e' and '__DATA__' What's wrong?

Re: $. - smarter than you might think

Many Thanks to...

First and foremost, choroba presented the idea in chatterbox.

shmem prodded me to write the "Why It Works" section, and also provided two of the "See also" links.

ambrus reminded us how to overwrite just one slot in a typeglob.

In reply to Using the DATA file handle for ARGV by samwyse

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