The first two things that come to my mind:

$_ = "some string with multiple words";
@first_three = (split)[0..2];
  # or
($first_three) = /^\s*(\S+\s+\S+\s+\S+)/;
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Golf swings on...

$str = 'yo ho ho and a bottle crumbs.
hangin and bangin in the upper-class slums.';

print join ' ', (split /\s+/, $str)[0..4];
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Time for some Perl Golf:

$str=
    'yo ho ho and a bottle crumbs. 
hangin and bangin in the upper-class slums.';

@word = ($str =~ /(\w+)/g);

print join ':', @word[0..4];
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Here's a slightly tested solution:

my $str="hello there bob smith and more words";
my $num_words=5;
if (my ($words) = $str=~ /((?:\w+(?:\W+|$)){$num_words})/) {
 print $words;
}
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@words = split /\W+/, $words;
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