use strict;
use warnings;
use Data::Dump qw(pp);

my %oldvalue = (
    foo => "1",
    bar => "2",
    baz => "3"
);

my %newvalue = (
    foo => "A"
);


%oldvalue = (%oldvalue, %newvalue);
pp \%oldvalue;      # { bar => 2, baz => 3, foo => "A" }