G'day RenMcCourtey,

As a more general solution to those already provided, when confronted with blocks of data that are to be processed as a single unit, read each block as a single record by localising $/, the input record separator. (See "perlsub: Temporary Values via local()" and "perlvar: Variables related to filehandles".)

Here's how you might do this with your posted data and requested output format:

#!/usr/bin/env perl -l

use strict;
use warnings;

{
    local $/ = "\ndn: ";

    while (<DATA>) {
        /
            ^ cn: \s+ (.*?) $
            .*?
            ^ orclnetdescstring: \s+ (.*?) $
        /msx;
        print join '=', $1, $2;
    }
}

__DATA__
dn: distinguished_name1
cn: common_name1
orclnetdescstring: complex_address_line1

dn: distinguished_name2
cn: common_name2
orclnetdescstring: complex_address_line2

dn: distinguished_name3
cn: common_name3
orclnetdescstring: complex_address_line3
[download]

Output:

common_name1=complex_address_line1
common_name2=complex_address_line2
common_name3=complex_address_line3
[download]