$/ = "";  # input record separator set to empty string

my $last_rec;
while (<>) {  # each record is terminated by /\n{2,}/
    if ( /^def/ ) {
        print $last_rec.$_ if ( $last_rec );
    }
    else {
        $last_rec = $_;
    }
}

##</code><code>##

$/ = "";
my $last_prefix = my $last_target = '';
while (<>) {
    if ( /^abc/ ) {
        print $last_prefix.$last_target if ( $last_target );
        $last_prefix = $_;
        $last_target = '';
    }
    elsif ( /^def/ ) {
        $last_target = $_;
    }
    elsif ( $last_target ) {
        $last_target .= $_;
    }
    elsif ( $last_prefix ) {
        $last_prefix .= $_;
    }
}