Would it be possible to change the values of the returns from the subs if the subs approach is used.

Yes, by doing some tricks with the symbol table:

use My::Config;
no strict 'refs';
*'My::Config::DB_NAME' = sub { 'new_db' };
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Are there any other factors that should be considered when making the choice.

Under certain conditions, subs declared with a prototype of () are considered for inlining (that's how use constant is implemented), which would make the sub method as fast as a hard-coded value. This is true as long as you don't call the sub as a class method. Example (which is tested on perl 5.8.0):

$ perl -MO=Deparse -e '
> package Foo;
> sub bar () { "baz" }
> package main;
> print Foo::bar(), "\n";
> print Foo->bar(), "\n";
> '
-e syntax OK
print 'baz', "\n";
print 'Foo'->bar, "\n";
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