A solution using split, array slices and shift. No idea if it is fast or slow as I haven't run any benchmarks.

use 5.026;
use warnings;

my $text = q{this is the text to play with};

for ( 1 .. 8 )
{
    say qq{$_-word ngrams of '$text'};
    say for nGramWords( $_, $text );
    say q{-} x 20;
}

sub nGramWords
{
    my( $nWords, $string ) = @_;

    my @words = split m{\s+}, $string;
    my $start = 0;
    my @nGrams;

    while ( scalar @words >= $nWords )
    {
        push @nGrams, join q{ },
           qq{START INDEX: @{ [ $start ++ ] } : },
           @words[ 0 .. $nWords - 1 ];
        shift @words;
    }

    return @nGrams;
}
[download]

The output.

1-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this
START INDEX: 1 :  is
START INDEX: 2 :  the
START INDEX: 3 :  text
START INDEX: 4 :  to
START INDEX: 5 :  play
START INDEX: 6 :  with
--------------------
2-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this is
START INDEX: 1 :  is the
START INDEX: 2 :  the text
START INDEX: 3 :  text to
START INDEX: 4 :  to play
START INDEX: 5 :  play with
--------------------
3-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this is the
START INDEX: 1 :  is the text
START INDEX: 2 :  the text to
START INDEX: 3 :  text to play
START INDEX: 4 :  to play with
--------------------
4-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this is the text
START INDEX: 1 :  is the text to
START INDEX: 2 :  the text to play
START INDEX: 3 :  text to play with
--------------------
5-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this is the text to
START INDEX: 1 :  is the text to play
START INDEX: 2 :  the text to play with
--------------------
6-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this is the text to play
START INDEX: 1 :  is the text to play with
--------------------
7-word ngrams of 'this is the text to play with'
START INDEX: 0 :  this is the text to play with
--------------------
8-word ngrams of 'this is the text to play with'
--------------------
[download]

I hope this is of interest.