#!/usr/local/bin/perl
use strict;
use warnings;
use Data::Dumper;

sub recurseHash{
  my ($targetHash, $mergeHash) = @_;
  while( my ($key, $val) = each %{$targetHash} ){
    if( ref($val) eq 'HASH' ){
      if( !exists($mergeHash->{$key}) ){
          mergeHash->{$key} = {};
      }
      &recurseHash($targetHash->{$key}, $mergeHash->{$key});
      next;
    }
    mergeHash->{$key} = $val;
  }
}

my %a = &recurseHash({a=>'a'}, {b=>'b'});
warn Dumper \%a;