So, I am getting the point that taint, and later versions of Perl are trying to make it difficult to use relative paths for modules!

You may put it like that. It turned out that too many people get it wrong and catch security holes, so making it difficult (but not impossible) gives people a chance to ponder over other approaches.

If a website has more than one environment, then you need a plan anyway (again, nothing to do with taint mode) how you deploy and maintain the files in your different environments. There are many solutions for that, but I'd go for something like this:

/home/myusername/somewebsite/prod/cgi-bin
/home/myusername/somewebsite/prod/lib
/home/myusername/somewebsite/prod/templates
[download]

with the same subdirectories for dev and test. So each environment has its own base directory, but below that they all have the same structure. Then it is indeed possible to use FindBin to detect which environment you're actually in (assuming you don't run a persistent interpreter like mod_perl).

my ($prefix,$website,$environment,$basedir);
BEGIN {
   $prefix = '/home/myusername';
   $website = 'somewebsite';
   use FindBin qw($RealBin);
   if ($RealBin =~ m!$prefix/$website/(dev|test|prod)/cgi-bin!) {
      $environment = $1; # This is now untainted!
      $basedir = "$prefix/$website/$environment";
   }
   else die "Bad or no environment '$1'";
}
use lib "$basedir/lib";
my $tt = Template->new({INCLUDE_PATH => "$basedir/templates"});
...;
[download]

The BEGIN block is needed to do the necessary calculations during the compilation so that the directory is available when use lib is processed.

Other alternatives include setting the environment as an environment (sic!) variable in the corresponding section of the web server config. Environment variables are tainted, so again you need to validate/untaint them.

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