examples/modules/namespace.pl
#!/usr/bin/perl
use strict;
use warnings;

require "examples/modules/namespace_lib.pl";

print Calc::add(3, 4), "\n";

examples/modules/namespace_lib.pl
package Calc;
use strict;
use warnings;

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;

Here, within the Calc namespace you don't have to use the full
name, only when you are using it outside.