It seems you want a window x words big?

@words=$text_body =~ /(\S+)/g;
#compute $start and $end
$newtext=join(" ",@words[$start..$end]);
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Update:Without copying. Note: I could not get the foreach deal to work at all so had to use while with a counter.

# $start and $window

$i++ while($i<$start && /(\S+\s+)/g);

$i=0;
while(/\G(\S+\s+)/g && $i<$window_length)
{
    $newtext .= "$1";
    $i++;
}
[download]

Note also: Two types of while. Probably should pick one you like the best and standardize on that.

This skips the $start number of words and the assigns $window_length words to $newtext preserving whitespace

Another Update: $start of 0 was not working but reversing the tests in the first while fixes that.