Learning Perl, 5th Edition

Filehandles

A filehandle is the name in a Perl program for an I/O connection between your Perl process and the outside world. That is, it’s the name of a connection, not necessarily the name of a file.

Filehandles are named like other Perl identifiers (with letters, digits, and underscores, but they can’t start with a digit), though since they don’t have any prefix character they might be confused with present or future reserved words, or with labels, which we cover in Chapter 10. Once again, as with labels, the recommendation from Larry is that you use all uppercase letters in the name of your filehandle—not only will it stand out better, but it will also guarantee that your program won’t fail when a future (lowercase) reserved word is introduced.

But there are also six special filehandle names that Perl already uses for its own purposes: STDIN, STDOUT, STDERR, DATA, ARGV, and ARGVOUT.^[*] Although you may choose any filehandle name you’d like, you shouldn’t choose one of those six unless you intend to use that one’s special properties.^[†]

Maybe you recognized some of those names already. When your program starts, STDIN is the filehandle naming the connection between the Perl process and wherever the program should get its input, known as the standard input stream. This is generally the user’s keyboard unless the user asked for something else to be the source of input, such as a file or the output of another program through a pipe.^[‡] There’s also the standard output stream, which ...