How did you check that the array gets destroyed when the control leaves your function?

use strict;

my %g_User;
sub setup_user_details {
  my ($username) = @_;
  my @l_UserDetails = ("John","Doe");
  $g_user{$username} = \@l_UserDetails;
};

setup_user_details('user1');
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Your naming convention is a bit unwieldly, but I attribute that to your company coding guidelines. In Perl it's conventional to use lower_caps_names instead of CamelCase, and prefixing of variables with their scope is uncommon because the strict pragma enforces proper scoping.