$ cat foo.pl
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $hr = {
    KEY11=>{a=>1, b=>2, c=>3, d=>4},
    KEY12=>{a=>10, b=>20, c=>30, d=>40}
};

@{$$hr{KEY11}}{qw(a b)} = @{$$hr{KEY12}}{qw(c d)};

print Dumper($hr);