Yet another approach that would work for the kind of data you posted: 
{
    local $/ = '***END***';
    while (<>) {
       print if ( /mykeyword2|mykeyword4/ );
    }
}

[download]
That sets perl's "input record separator" to be the end-of-record string, instead of the default end-of-line string ("\n" or "\r\n", depending on your OS). In the version shown above, the line-termination character(s) following each "***END***" will be included at the beginning of the next record. If you prefer (and if you know for sure that your input data will always use the same style of line-termination), you can set $/ like this: 
     local $/ = "***END***\n";  # or "***END***\r\n"

[download]
UPDATE:

Having seen AM's riz's reply below, I have to assume that s/he didn't understand what I said, so here's a full, tested version of the approach I described: 

#!/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***

[download]
Note that when $/ is set to some non-default value, the "chomp" function uses that value to remove the record delimiter string from the end its operand ($_ in this case).

In reply to Re: searching data lines between keywords by graff
in thread searching data lines between keywords by riz

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