How can I use an external lib in a BEGIN block - perl

Question

I want to use an external library in the BEGIN block of a Perl script.
The first test which I did was to check whether my #INC is getting popullated if I push some values:
use strict;
use warnings;
BEGIN {
push #INC, "d:/external_pm/";
use Data::Dumper;
print Dumper #INC;
}
Which works as expected and shows:
$VAR1 = 'D:/perl/5163/site/lib';
$VAR2 = 'D:/perl/5163/lib';
$VAR3 = '.'; # I am not sure about this one?!
$VAR4 = 'd:/external_pm/';
Now I want to import a module right after the push:
use strict;
use warnings;
BEGIN {
push #INC, "d:/external_pm/";
use Data::Dumper;
print Dumper #INC;
use ExtScript;
}
The error which follows is showing me that #INC was not updated:
Can't locate ExtScript.pm in #INC (#INC contains: D:/perl/5163/site/lib
D:/perl/5163/lib .) at file.pl line 9.
BEGIN failed--compilation aborted at file.pl line 9.
Why the #INC it's not updating?
I can't import a module in the BEGIN block? Or is a missusage of Perl?

Answer 1

use statements are executed at compile time (specifically, during the BEGIN phase), whereas normal code is run later. Let's look at this simplified snippet:
BEGIN {
push #INC, "some/dir";
use Example;
}
If we spell out all phases explicitly, that would be equivalent to:
BEGIN {
push #INC, "some/dir";
BEGIN { require Example; Example->import() }
}
So the Example module will be imported before the push runs.
There are a number of ways to address this.
The simplest way is to put only the #INC manipulation into a BEGIN block, and import the module outside:
BEGIN { push #INC, "some/dir" }
use Example;
A better solution is to use the lib pragma to handle #INC:
use lib "some/dir";
use Example;
However, there is a major difference: use lib puts additional directories at the beginning of the module search path, so you may accidentally override other modules. push #INC only adds directories to the end, as a fallback if a module wasn't found in other locations.

Answer 2

use ExtScript; is executed before push #INC. Either
move use ExtScript; out of the BEGIN block
or change it to require/import
or use the -I command-line option or PERL5LIB env variable.