What is the role of the BEGIN block in Perl?

Question

I know that the BEGIN block is compiled and executed before the main body of a Perl program. If you\'re not sure of that just try running the command perl -cw over this:

Answer 1

While the other answers are true, I find it also worth to mention the use of BEGIN and END blocks when using the -n or -p switches to Perl.

From http://perldoc.perl.org/perlmod.html

When you use the -n and -p switches to Perl, BEGIN and END work just as they do in awk, as a degenerate case.

For those unfamiliar with the -n switch, it tells Perl to wrap the program with:

while (<>) {
    ...  # your program goes here
}

http://perldoc.perl.org/perlrun.html#Command-Switches if you're interested about more specific information about Perl switches.

As an example to demonstrate the use of BEGIN with the -n switch, this Perl one-liner enumerates the lines of the ls command:

ls | perl -ne 'BEGIN{$i = 1} print "$i: $_"; $i += 1;'

In this case, the BEGIN-block is used to initiate the variable $i by setting it to 1 before processing the lines of ls. This example will output something like:

1: foo.txt
2: bar.txt
3: program.pl
4: config.xml