Writing Your First Perl Script

A one-off program is fine, but the point of Perl is small, reusable tools. This walks through building one.

The shebang line

The first line tells the system how to run the file:

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

#!/usr/bin/env perl finds Perl wherever it lives on the machine — more portable than hard-coding a path.

Reading command-line arguments

Arguments arrive in the special array @ARGV:

my ($name) = @ARGV;
$name //= "world";   # default if none given
say "Hello, $name!";

Run it:

perl greet.pl Ada   # Hello, Ada!
perl greet.pl       # Hello, world!

Reading standard input

To process piped input or a file line by line, read from the <STDIN> diamond:

while (my $line = <STDIN>) {
    chomp $line;        # remove the trailing newline
    say "got: $line";
}

cat names.txt | perl greet.pl

A complete, useful example

A script that counts non-blank lines in whatever you pipe to it:

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

my $count = 0;
while (my $line = <STDIN>) {
    next if $line =~ /^\s*$/;   # skip blank lines
    $count++;
}
say "$count non-blank lines";

Making it executable

So you can run it directly instead of typing perl each time:

chmod +x count.pl
./count.pl < notes.txt

Where to go next

Learn to open and write files properly in Working with Files and I/O, and compress whole scripts into Perl One-Liners.