I am by no means a perl wizard, but I have run into this problem many times. My solution is to stick all my constants into a config file of the format:

KEY=VALUE
KEY=VALUE
...
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if is then really easy to read the values in

open(F,"config.conf");
my %config = {}'
while(<F>) {
  (my $key,$value) = split /=/;
  $config{"$key"} = $value;
}
close(F)
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if you stick this in an object and only reference the object through some function ie: $obj->v('KEY'); then it is unlikely the value will get modified, it is always possible they can modify your "constants" through  $Packagename::config{'key'} but good programing style should dictate that this shouldn't happen.