#!/usr/bin/perl

use strict;

my @keepers;
{
    local $/ = '***END***';
    while ( <DATA> ) {
        next unless ( /^\s*mykeyword[24]/ );
        chomp;
        push @keepers, $_;
    }
}
print join '', @keepers;

__DATA__
mykeyword1
several data lines
containing junk
***END***
mykeyword2
several lines
containing target data
***END***
mykeyword3
several lines
containing junk
***END***
mykeyword4
again several lines
containing target data
***END***
mykeyword1
several data lines
containing junk
***END***
mykeyword2
several lines
containing target data
***END***
mykeyword3
several lines
containing junk
***END***
mykeyword4
again several lines
containing target data
***END***