my @filtered = grep { !m/(?:mykeyword|\*\*\*END\*\*\*)/ } <DATA>;
print @filtered;

__DATA__
mykeyword1
<several data lines>
***END***
mykeyword2
<several lines>
***END***
mykeyword3
<several lines>
***END***
mykeyword4
<again several lines>
***END***