I don't suppose this satisfies your criteria? It works, but would screw up if you tried to nest uses of it.

#! perl -sw
use strict;

{
    my $next=0;
    sub elements(\@$) {
        my ($ref, $count) = @_;
        my $max = $#$ref;
        $next=0, return if $next > $max;
        my $first = $next;
        my $last = $next+$count-1;
        $last = $max if $max < $last;
        $next += $count;
        @{$ref}[$first .. $last];
    }
}

my @letters = ('a'..'z');
while(my @els = elements(@letters, 5) ) {
    print "@els\n";
}
__END__
a b c d e
f g h i j
k l m n o
p q r s t
u v w x y
z
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