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

# original hash
my $h = {
 mm => 'July',
 yyyy => '1975',
 dd => '31',
 name => 'Milo',
 last_name => 'Manara',
};

print Dumper($h);

my $x = { mm=>'month', dd=>'day', yyyy=>'year' };
my $t;

@$t{map{$$x{$_}||$_}keys %$h) = values %$h;

print Dumper($t);

##</code><code>##

NBKI44V@B000F1FA4379F /Work/Tools/DataMap
$ ./rehash2.pl
$VAR1 = {
          'mm' => 'July',
          'name' => 'Milo',
          'dd' => '31',
          'last_name' => 'Manara',
          'yyyy' => '1975'
        };
$VAR1 = {
          'month' => 'July',
          'day' => '31',
          'name' => 'Milo',
          'year' => '1975',
          'last_name' => 'Manara'
        };