use array slices in combination with hash slices, thusly:

use strict;

my @keys = keys %hash;
my $span = 20;

my ($page, $last_page) = (0, int($#keys / $span));
while ($last_page > 0 and $i < $last_page) {
  make_page($page, 
            @hash{@keys[$i * $span .. (++$i) * $span - 1]});
}
make_page($last_page, 
          @hash{@keys[$last_page*span .. $#keys]});
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