It can be done as follows:-

#!/usr/bin/perl -w
use strict;
use warnings;
my @arry=(18,96,117,336.9);
my %old_hash=(
    a   =>  {
        aa  =>  {
            aaa =>  "aaa_value",
            aab =>  "aab_value",
            aac =>  "aac_value",
            aad =>  "aad_value",
        },
        ab  =>  {
            aba =>  "aba_value",
            abb =>  "abb_value",
            abc =>  "abc_value",
            abd =>  "abd_value",
        },
        ac  =>  {
            aca =>  "aca_value",
            acb =>  "acb_value",
            acc =>  "acc_value"
        },
        ad  =>  \@arry
    },
    b   =>  {
        ba  =>  {
            baa =>  "baa_value",
            bab =>  "bab_value",
            bac =>  "bac_value",
            bad =>  "bad_value",
        },
        bb  =>  {
            bba =>  "bba_value",
            bbb =>  "bbb_value",
            bbc =>  "bbc_value",
            bbd =>  "bbd_value",
        },
        bc  =>  {
            bca =>  "bca_value",
            bcb =>  "bcb_value",
            bcc =>  "bcc_value"
        },
        db  =>  [5,8,69,99]
    }
);
my %hash=%old_hash;
my $thash=&process_hash(\%hash);
my %new_hash=%{$thash};
use Data::Dumper;
print "Original\n".Dumper(\%old_hash);
print qq{\n\n------------------------------------------------------------------------------\n\n};
print "Modified\n".Dumper(\%new_hash);
sub process_hash {
    my ($temp_hash)=@_;
    if(&is_hash($temp_hash)) {
        foreach my $key(keys %{$temp_hash}) {
            if(&is_hash($temp_hash->{$key})) {
                my %t=%{$temp_hash->{$key}};
                $temp_hash->{$key}=&process_hash(\%t);
            } elsif(&is_array($temp_hash->{$key})) {
                my @t=@{$temp_hash->{$key}};
                $temp_hash->{$key}=&process_hash(\@t);
            } else {
                if($temp_hash->{$key} && $temp_hash->{$key} ne '') {
                    $temp_hash->{$key}="--".$temp_hash->{$key}."--";
                }
            }
        }
    } elsif(&is_array($temp_hash)) {
        my $x=0;
        foreach my $values(@$temp_hash) {
            if(&is_array($values)) {
                my @t=@{$values};
                $temp_hash->[$x]=&process_hash(\@t);
            } elsif(&is_hash($values)) {
                my %t=%{$values};
                $temp_hash->[$x]=&process_hash(\%t);
            } else {
                $temp_hash->[$x]="--".$values."--";
            }
            $x++;
        }
    }
    return $temp_hash;
}
sub is_hash {
    my ($ref) = @_;
    return 0 unless ref $ref;
    if ( $ref =~ /^HASH/ ) {
        return 1;
    } else {
        return 0;
    }
}
sub is_array {
    my ($ref) = @_;
    return 0 unless ref $ref;
    eval {
        my $a = @$ref;
    };
    if ($@=~/^Not an ARRAY reference/) {
        return 0;
    } elsif ($@) {
        die "Unexpected error in eval: $@\n";
    } else {
        return 1;
    }
}

Output:-

Original
$VAR1 = {
          'a' => {
                   'ab' => {
                             'abb' => 'abb_value',
                             'abc' => 'abc_value',
                             'aba' => 'aba_value',
                             'abd' => 'abd_value'
                           },
                   'ad' => [
                             18,
                             96,
                             117,
                             '336.9'
                           ],
                   'ac' => {
                             'acb' => 'acb_value',
                             'aca' => 'aca_value',
                             'acc' => 'acc_value'
                           },
                   'aa' => {
                             'aaa' => 'aaa_value',
                             'aad' => 'aad_value',
                             'aac' => 'aac_value',
                             'aab' => 'aab_value'
                           }
                 },
          'b' => {
                   'db' => [
                             5,
                             8,
                             69,
                             99
                           ],
                   'ba' => {
                             'bad' => 'bad_value',
                             'baa' => 'baa_value',
                             'bab' => 'bab_value',
                             'bac' => 'bac_value'
                           },
                   'bb' => {
                             'bbc' => 'bbc_value',
                             'bbb' => 'bbb_value',
                             'bbd' => 'bbd_value',
                             'bba' => 'bba_value'
                           },
                   'bc' => {
                             'bcc' => 'bcc_value',
                             'bca' => 'bca_value',
                             'bcb' => 'bcb_value'
                           }
                 }
        };


------------------------------------------------------------------------------

Modified
$VAR1 = {
          'a' => {
                   'ab' => {
                             'abb' => '--abb_value--',
                             'abc' => '--abc_value--',
                             'aba' => '--aba_value--',
                             'abd' => '--abd_value--'
                           },
                   'ad' => [
                             '--18--',
                             '--96--',
                             '--117--',
                             '--336.9--'
                           ],
                   'ac' => {
                             'acb' => '--acb_value--',
                             'aca' => '--aca_value--',
                             'acc' => '--acc_value--'
                           },
                   'aa' => {
                             'aaa' => '--aaa_value--',
                             'aac' => '--aac_value--',
                             'aad' => '--aad_value--',
                             'aab' => '--aab_value--'
                           }
                 },
          'b' => {
                   'ba' => {
                             'baa' => '--baa_value--',
                             'bad' => '--bad_value--',
                             'bab' => '--bab_value--',
                             'bac' => '--bac_value--'
                           },
                   'db' => [
                             '--5--',
                             '--8--',
                             '--69--',
                             '--99--'
                           ],
                   'bb' => {
                             'bbb' => '--bbb_value--',
                             'bbc' => '--bbc_value--',
                             'bba' => '--bba_value--',
                             'bbd' => '--bbd_value--'
                           },
                   'bc' => {
                             'bcc' => '--bcc_value--',
                             'bca' => '--bca_value--',
                             'bcb' => '--bcb_value--'
                           }
                 }
        };