I've had similar situations where I had to "just get it working", and, seeing the variance in environment settings across systems, I just did something like this:

my $username = $ENV{USER} || $ENV{LOGNAME} || `whoami`;
chomp $username;  # in case it came from the shell command
[download]

But I never had to do this in a situation where "su" was being used, and I'd worry about whether one or the other %ENV setting gets "updated" or not across the "su" (with or without the "-" to invoke the new user's rc file(s)).

Having just tried it on freebsd, it looks like "whoami" definitely reports the "effective username" (i.e. the user that you "su" to); maybe the price of running the backtick command is worthwhile, considering the stability you get in return...