#!/usr/bin/perl

use strict;

local $/ = "\n\n";

while( <DATA>) {
    my ($line) = $_ =~ /\b(start\s.*\n?.*end)\b/;
    $line =~ s/\n/ /g if ( $line );
    print "$line\n" if ( $line );
}

__DATA__
asdasd start asdasd asdasdasd asdasdas end asdasdas

adasdas start as asdas dasdasdad asdasddas end qweqwe

asdasd start asdsadsdasddasds sdasdas asdasdasdasd asdasdsa
asdasd asdasdasd end

ds start

asda end