#!/usr/bin/perl -w
use strict;

while (<DATA>)
{
   next if /^\s*$/; #skip blank lines
   chomp;           #optional as \n would get deleted anyway
   s/^.*?start\s+//; #remove start and all before
   s/end.*//;       #remove end and all after
   print "$_\n";
}

#prints:
# asdasd asdasdasd asdasdas 
# as asdas dasdasdad asdasddas 
# asdsadsdasddasds sdasdas asdasdasdasd asdasdsa asdasd asdasdasd 

__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