#!usr/bin/perl
use strict;
use warnings;

local $_ = <<ENDE;
<OrderPosAd Datum="2005-02-28" Bla="blubb">hjgadf</OrderPosAd>
<Suppe>Grünkern</Suppe>
<OrderPosAd Datum="2005-02-27" Bla="blubber">hjishfgadf</OrderPosAd>
<Fleisch>Putenbrust</Fleisch>
<OrderPosAd Datum="2005-02-26" Bla="blasenwerf">hjoejwhishfgadf</OrderPosAd>
ENDE

my @gefunden;

while (s~<OrderPosAd([^<>]*)>[^<>]+</OrderPosAd>~~) {
    my $tag = $1;
    my ($merkmal) = $tag =~ / Datum=\"(.*?)\"/;
    push @gefunden, $merkmal;
}

print "@gefunden";