BEGIN块中的变量声明

Question

In a script like the following, is it possible without dropping 'my' to effectively declare 'var' only once and have it visible outside the BEGIN block?

echo -e "\n\n\n" | \
  perl -lne 'BEGIN { my $var="declared & initialized once" } print $var'

Also, why declaring var without 'my' makes it visible outside the BEGIN block?

Answer 1

Place a my $var; before the BEGIN block:

$ perl -le 'my $var; BEGIN { $var = "declared"; } print $var;'
declared

my gives the variable lexical scope, so $var is not defined in your example outside the BEGIN block. Removing the my effectively makes it a global variable, which is accessible across the script after assignment.

Answer 2

Also, why declaring var without 'my' makes it visible outside the BEGIN block?

You're not declaring it then. It is autodeclared as global, if you're not using use strict (which prevents the declaration by default). In a one-liner, strict hurts more than it helps; I'm perfectly fine with not doing a declaration in such a context.