/home/myusername/somewebsite/prod/cgi-bin
/home/myusername/somewebsite/prod/lib
/home/myusername/somewebsite/prod/templates
##</code><code>##

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"});
...;