You probably want to do something like:

my $a = qq(foo bar foo bar foo bar foo bar foo bar);
$a =~ s/((\w+\s+){4}).+/$1.../g; 
print $a;
__END__
output:
foo bar foo bar ...
[download]

This way, any words past the fourth are replaced by "...", and if there are fewer than four words no replacement happens.

my @words = split / /, $string, 6;
$words[-1] = '...' if @words==6;
$string = join ' ', @words;
[download]

That's kind of an odd requirement. Is this homework? It makes more sense to just extract the first five words and then append whatever you want. The way you posed it awkward and should be reframed.

Hmmm... on further thought the expanded code with the conditional really just looks like a plain replacement. I've added a second code block where I turn this into a substitution.

The original code. This will clobber $text if there are less than five words.

 # Capture the first five words including whitespace.
 ($text) = $text =~ / (\S+ \s+
                       \S+ \s+
                       \S+ \s+
                       \S+ \s+
                       \S+ )/x;
 # Shrink any internal whitespace
 $text =~ s/\s+/ /g;
[download]

The amended code to compensate for the previous block's clobbering action

 # Capture the first five words including whitespace.
 if ( $text =~ / (\S+ \s+
                  \S+ \s+
                  \S+ \s+
                  \S+ \s+
                  \S+ )/x ) {
     # The regex extracted the first five words
     $text = "$1 ...";

     # Shrink any internal whitespace
     $text =~ s/\s+/ /g;
 }
[download]

That previous block really looked like a substition. Here it is, reframed again

 if ( $text =~ s/ (\S+ \s+
                   \S+ \s+
                   \S+ \s+
                   \S+ \s+
                   \S+ )/$1 .../x ) {
     # The match succeeded, shrink the white space
     $text =~ s/\s+/ /g;
 }
[download]

Updated Altered $text .= "whatever" to $text .= " ...". Also added caveats on word count

Seeking Green geeks in Minnesota

#!/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]

CU
Robartes-