use strict;
use Data::Dumper;

my %hash1 = qw/ A 1 B 2 /;
my %hash2 = qw/ C 3 D 4 /;

mergetwohashes(\%hash1, \%hash2);
print Dumper(\%hash1);

sub mergetwohashes {
    my ($a, $b) = @_;
    $a->{$_} = $b->{$_} foreach keys %$b;
}

##</code><code>##

$VAR1 = {
          'A' => '1',
          'B' => '2',
          'C' => '3',
          'D' => '4'
        };