Oops, I thought your 3rd line is just one big line... Didn't see that it was broken into 3rd and 4th line. Anyway, I modified the code... so, this should do the trick.

I'm not sure how to update my existing comment, that's why I had to create a new one.

#!/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
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