Here's another option with your data in the file data.txt:

use Modern::Perl;
use File::Slurp qw/read_file/;

my $lines;

for (read_file 'data.txt') {
    if ( /Orderbook/ .. /ISIN/ ) {
        $lines .= $_;
    }
    elsif ($lines) {
        my ( $id, $sy, $is ) =
          $lines =~ /ID:(.*)[\s\S]+Symbol:(.*)[\s\S]+ISIN:(.*)/;
        my $line = "$id:$sy:$is";
        $line =~ s/\s//g;
        say $line;
        $lines = '';
    }
}
[download]

Output:

QYQ:LUP2L100OHM:SE0004017929
R1M:TLS2K50OHM:SE0004018539
QNF:MINILONGOMXAO:SE0003990183
QX8:ALF2K160OHM:SE0004017440
NC0:BOL2K170OHM:SE0003842137
NEV:NOK2K90OHM:SE0003843069
[download]