How's this:

#!/usr/local/bin/perl -w
use strict;
my $text="Twenty centuries of stony sleep were vexed to nightmare";
my $numwords=5;
my $replacement="lead to a crick in the back.";
print $text;
$text=~ s/^((\w+\s){$numwords}).*$/$1.$replacement/e;
print $text;
__END__
Twenty centuries of stony sleep were vexed to nightmare
Twenty centuries of stony sleep lead to a crick in the back.
[download]

perlre is your friend.

CU
Robartes-