Slupring the whole file into memory at once tends to often suck. And avoiding that also means you can avoid complicated regexes (using a very SMoP instead).

my $message= '';
while( <> ) {
    if( ! /^\d{4}-\d\d-\d\d / ) {
        $message .= $_;
    } else {
        print $message
            if(  $message  =~ /keyword/  );
        $message= $_;
    }
}
print $message
    if(  $message  =~ /keyword/  );
[download]

Or, if you prefer:

my $line= <>;
while( defined $line ) {
    my $message= '';
    do {
        $message .= $line;
    } while(    defined( $line= <> )
            &&  $line !~ /^\d{4}-\d\d-\d\d /
    );
    print $message
        if(  $message  =~ /keyword/  );
}
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.