use strict;

my $text = do { local $/; <> };
my $date_rgx = qr/\d{4}-\d{2}-\d{2}/;

my ( $date, $text ) = ( '', '' );

for ( split /\n($date_rgx)/, $text ) {  # capture the date (but not the preceding \n)
    if ( /^$date_rgx$/ ) {
        $date = $_;
    }
    elsif ( /\bkeyword\b/ ) {
        print "$date$_\n";
    }
}