The function returns a hash. What you do with that hash is up to you. You haven't shown your program, so I can only give you general advice.

One approach would be to set up %ENV with the newly found values. Another one would be to merge %ENV with the newly found values, as you (and the example) show. A third approach would be to keep the two sets of values separate.

To simulate running under the login environment as closely as possible, %ENV = get_login_env() will certainly do that, but in the process, you lose any indication that your program is running as cron job. In general, I think the approach of merging %ENV and the values from the login environment, with the values from the login environment taking precedence is a good one.

Well, it works from the prompt but not via cron.

use strict;
use warnings;
use MIME::Lite;

# from the link you gave
sub get_login_env {
  local %ENV;
  my $shell = shift || (getpwuid($<))[8];
  my $env = `echo env | perl -e 'exec {"$shell"} -sh'`;
  if (wantarray) {
    my @pieces = ( $env =~ m/^(.*?)=((?:[^\n\\]|\\.|\\\n)*)/gm );
    s/\\(.)/$1/g foreach @pieces;
    return @pieces;
  }
  else {
    return $env;
  }
}

%ENV = (%ENV, get_login_env());
my $mailvar = $ENV{'TESTMAILUSRS'}; 
my @mailUsers;
my $msg = new MIME::Lite();
$msg->build(
  From            =>    'us@here.com',
  Subject        =>    'Email from perl via cron',
  Type            =>    'text/html',
  Data            =>    'testing',
    Debug            => 1

);

for (split /[\s\n]/, $mailvar){
    if($_){
        $msg->add(To    =>    $_);
    }
}

eval { $msg->send };
die "MIME::Lite->send failed: $@\n" if $@;
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How does it fail for you? Does sh pick up the correct environment? Maybe you set some environment variables in another rc file, or in .profile or .kshrc. Start some debugging.