6.1 Input from STDIN

Reading from standard input (via the Perl filehandle called STDIN) is easy. We've been doing it already with the <STDIN> operation. Evaluating this in a scalar context gives the next line of input,[1] or undef if there are no more lines, like so:

$a = <STDIN>; # read the next line

[1] Up to a newline, or whatever you've set $/ to.

Evaluating in a list context produces all remaining lines as a list: each element is one line, including its terminating newline. We've seen this before, but as a refresher, it might look something like this:

@a = <STDIN>;

Typically, one thing you want to do is read all lines one at a time and do something with each line. One common way to do this is:

while (defined($line = <STDIN>)) {
    # process $line here
}

As long as a line has been read in, <STDIN> evaluates to a defined value, so the loop continues to execute. When <STDIN> has no more lines to read, it returns undef, terminating the loop.

Reading a scalar value from <STDIN> into $_ and using that value as the controlling expression of a loop (as in the previous example) occurs frequently enough that Perl has an abbreviation for it. Whenever a loop test consists solely of the input operator (something like <...>), Perl automatically copies the line that is read into the $_ variable.

while (<STDIN>) { # like "while(defined($_ = <STDIN>)) {"
    chomp; # like "chomp($_)"
    # other operations with $_ here
}

Since the $_ variable is the default for many operations, you can save a noticeable amount of typing this way.