#!/usr/bin/env perl
use Modern::Perl;

local $/ = ""; # split records on a blank line

while (<DATA>) {
    if ( m[Orderbook\ ID: \s* (.+?) \n
	          \s* Symbol: \s* (.+?) \n
	          \s*   ISIN: \s* (.+?) \n ]xs ){
		say "$1:$3:$2";

		# comment the previous line and uncomment the next three if you really
		# want to strip whitespace from your values, unlike your first
		# example

		# my $l = "$1:$3:$2";
		# $l =~ s/\s+//g;
		# say $l;
	}
}

__DATA__
[Sample data removed by request of the original poster.]