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;
}