...
sub groupsOf (&$@);
...
sub groupsOf (&$@)
{
    my $rcToRun  = shift;
    my $groupsOf = shift;

    my $rcDoIt;
    $rcDoIt = sub
       {
           $rcToRun->( map shift, 1 .. min scalar( @_ ), $groupsOf ),
           @_ ? &$rcDoIt : ();
       };
    &$rcDoIt;
}
...

##</code><code>##

use strict;
use warnings;

use feature qw{ say };
use List::Util qw{ min };

sub groupsOf (&$@);

my @arr    = ( 1 .. 20 );
say qq{@arr};

my @twos   = groupsOf { reverse @_ } 2, @arr;
say qq{@twos};

my @threes = groupsOf { reverse @_ } 3, @arr;
say qq{@threes};

say q{-} x 40;

my @rot1 = groupsOf { rotateFromFront( 2, @_ ) } 5, @arr;
say qq{@rot1};

my @rot2 = groupsOf { rotateFromBack( 2, @_ ) } 5, @arr;
say qq{@rot2};

say q{-} x 40;

my @sums = groupsOf { my $sum; $sum += $_ for @_; $sum } 6, @arr;
say qq{@sums};

sub groupsOf (&$@)
{
    my $rcToRun  = shift;
    my $groupsOf = shift;

    my $rcDoIt;
    $rcDoIt = sub
       {
           $rcToRun->( map shift, 1 .. min scalar( @_ ), $groupsOf ),
           @_ ? &$rcDoIt : ();
       };
    &$rcDoIt;
}

sub rotateFromBack
{
    my( $places, @arr ) = @_;
    unshift @arr, pop @arr for 1 .. $places;
    return @arr;
}

sub rotateFromFront
{
    my( $places, @arr ) = @_;
    push @arr, shift @arr for 1 .. $places;
    return @arr;
}

##</code><code>##

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2 1 4 3 6 5 8 7 10 9 12 11 14 13 16 15 18 17 20 19
3 2 1 6 5 4 9 8 7 12 11 10 15 14 13 18 17 16 20 19
----------------------------------------
3 4 5 1 2 8 9 10 6 7 13 14 15 11 12 18 19 20 16 17
4 5 1 2 3 9 10 6 7 8 14 15 11 12 13 19 20 16 17 18
----------------------------------------
21 57 93 39