johngg@shiraz:~/perl/Monks > perl -Mstrict -Mwarnings -E '
my @strs = (
   q{Mary had a little lamb},
   q{  its fleece was white as snow},
   );
my $nWords = 3;
my $qrWordStart = qr{(?x)
   (?: (?<= \A ) | (?<= \W ) )  # Preceded by either beginning of
                                #   string or non-word character
   (?= \w )                     # Followed by a word character
   };

foreach my $str ( @strs )
{
    my @posns;
    push @posns, pos $str
       while $str =~ m{$qrWordStart}g;
    say  qq{->@{ [ substr $str, 0, $posns[ $nWords ] ] }<-};
}'
->Mary had a <-
->  its fleece was <-