Take a look at Win32::TieRegistry. The following code is untested, but should append "appended data" to the 'bar' string value located under the HKEY_LOCAL_MACHINE\Software\foo key.

use Win32::TieRegistry;

my $reg = Win32::TieRegistry->new();
$reg->Delimeter('/');

my $key = $reg->{"LMachine/Software/foo/"};
my $val = $key->{"/bar"};

$key->{"/bar"} = $val . "appended data";
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