Some commentary on your posted code:

• my %BB_ENV = `/bin/sh . /usr/local/env.sh`;

  Backticks (`STRING`) return a simple scalar. Assigning this to a hash gives you a single, very long key associated with an undefined value, and will throw a warning when you use warnings (You are using warnings, right?). In order to obtain a set of key-value pair, you need to process that string. Assuming an output like printenv, you might use a pair of splits, like

  my %BB_ENV;
  for my $line (split /\n/, `/bin/sh . /usr/local/env.sh`) {
      my ($key,$value) = split /=/, $line, 2;
      $BB_ENV{$key} = $value;
  }
  [download]

• %ENV = { %ENV, %BB_ENV };

  A pair of curly brackets gives you an anonymous hash reference, not a hash. A hash ref is a scalar - see perlreftut. You probably mean just a simple list assignment, which would use parentheses:

  %ENV = ( %ENV, %BB_ENV );

  though I would instead manually copy in values, key by key:

  $ENV{$_} = $BB_ENV{$_} for keys %BB_ENV;

In any case, you should probably follow Anonymous Monk's links above for some other approaches to solving this issue.