examples/modules/library.pl
$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;
examples/modules/perl4_app.pl
#!/usr/bin/perl

require "examples/modules/library.pl";

print add(19, 23);
print "\n";
print "$base\n";
The 1; at the end of the library is needed in order to make sure the
compilation of library.pl returns a true value.

Otherwise one could get an error such as this one:
examples/modules/library.pl did not return a true value