#!perl

use strict;
use warnings;
my $para = 'Paragraph2:';

while (<DATA>) {
	print if (/^ {5}$para:?\s*$/i ... /^ {5}\S/ and /^ {7}/) 
}

__DATA__
     Paragraph1: 
         text11
     Paragraph2:
         text21
         text22
         text23
     Paragraph3:
         text31