Use of / /../ / operator is a good idea!

But here I just show one simple approach that is easy to debug. There are Lot's of ways to do what you need.

Update: I just assumed that the last line sequence was all one line as you had blank lines before other examples - maybe a bad assumption - bichonfrise74's code looks good to me also - there are many ways Rome here.

#!/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 asd
+asdasd end
[download]

Update:

sorry for goof, brain isn't working full speed today!

s/^.*?start/start/;
s/end.*/end/;
[download]

preserves start and end tokens.