Hmmm, fairly odd behavior. I think changing two lines will make it do what you expect:

# Was if ($$user_hash{SERVICE}->{$service}){
# Equivalent to if (${$user_hash}{SERVICE}->{$service}){
if (exists $user_hash->{SERVICE}{$service}) {
[download]

# Was my @users = @$user_hash{SERVICE}->{$service}->{USER};
my @users = @{ $user_hash->{SERVICE}{$service}{USER} };
[download]

Note that the proper way to access a hashref is $hash->{blah}.