As the number of functions we export grows, this soon can get out of hand. Especially for people who do not restrict the import by listing the functions to be imported. People who just write use Module::Name; are going to receive a growing number of function in their namespace. Many of those functions they will probably never use and they just pose a risk of colliding with functions from other sources.

For the module author at first it might have seemed to be a good idea to export everything. After all that makes it much easier to import everything, but later on it turned out that it would be probably better to export by default only the most commonly used functions and have a separate list of functions that the users can opt-in to import.

For that the import function of the Exporter module will look at two arrays. The @EXPORT array holds the list of functions that are imported by default and the @EXPORT_OK array holds the list of functions that can also be imported optionally.

So when the use write use Module::Name; without providing a list of functions, then everything listed in @EXPORT will be imported but nothing else.

Alternatively, the user can specify which functions to import by listing them after the module name: use Module::Name qw(this that); and then exactly those functions will be imported. Each one of the requested function names can come from either the @EXPORT array or the @EXPORT_OK array.

So given the B::Calc module in the B/Calc.pm file:

package B::Calc;
use strict;
use warnings;

use Exporter qw(import);

our @EXPORT = qw(add);
our @EXPORT_OK = qw(multiply);

my $base = 10;

sub add {
    validate_parameters(@_);

    my $total = 0;
    $total += $_ for (@_);
    return $total;
}

sub multiply {
}

sub validate_parameters {
    die 'Not all of them are numbers'
        if  grep {/\D/} @_;
    return 1;
}


1;

The user can write

use B::Calc;

and then only the add function will be imported.

The user can write:

use B::Calc qw(multiply);

and then only the multiply function will be imported but not the add function.

So once you start to list the function you'd like to import, you'll have to write down every function and you'll have to specify the exact list of functions to be imported.

Conclusion

@EXPORT is the list of functions to be imported by default.

@EXPORT_OK is the list of function to be imported optionally.