#! perl
use strict;
use warnings;
use Data::Dumper;

my %main_hash = (
                    keyA => [
                                { keya => 'vala' },
                                { keyb => 'valb' },
                                { keyc => 'valc' },
                                { keyd => 'vald' },
                            ],

                    keyB => [
                                { keye => 'vale' },
                                { keyf => 'valf' },
                                { keyg => 'valg' },
                                { keyh => 'valh' },
                            ],

                    keyC => [
                                { keyi => 'vali' },
                                { keyj => 'valj' },
                            ],
                );

print Dumper(%main_hash), "\n";

foreach my $k (sort keys %main_hash)
{
    my @array = @{ $main_hash{$k} };

    foreach my $i (1 .. $#array)
    {
        my %inner_hash_first = %{ $array[$i - 1] };
        my %inner_hash_next  = %{ $array[$i    ] };

        my ($first_key, $first_val) = each( %inner_hash_first );
        my ($next_key,  $next_val)  = each( %inner_hash_next  );

        print "Now compare ($first_key : $first_val) with ($next_key : $next_val)\n";
    }
}

##</code><code>##

Now compare (keya : vala) with (keyb : valb)
Now compare (keyb : valb) with (keyc : valc)
Now compare (keyc : valc) with (keyd : vald)
Now compare (keye : vale) with (keyf : valf)
Now compare (keyf : valf) with (keyg : valg)
Now compare (keyg : valg) with (keyh : valh)
Now compare (keyi : vali) with (keyj : valj)