Since you're making a separate copy of the list that gets passed in anyway, you could also use the splice trick that merlyn showed above, and get rid of all the arithmetic.

sub by_groups_of {
    my $by = shift;
    return sub { () } if ! $by || $by =~ /\D/ || ! @_;
    my @list = @_; # here's the copy. we're safe to splice

    return sub { splice @list, 0, $by };
}
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