Treat the ref like a normal scalar, no need to deref. Also it's my ($existing_users) = @_; (parens on the left for list context) or my $existing_users = shift;. See also perlreftut and perlref.

use warnings;
use strict;
use Data::Dumper;

my $existing_users = {
    'user1' => {
        'UserName' => 'user1',
        'Ignore' => 'N',
        'LastAccessDate' => '0000-00-00'
    },
    'user2' => {
        'UserName' => 'user2',
        'Ignore' => 'N',
        'LastAccessDate' => '0000-00-00'
    },
};

send_to_sub($existing_users);

sub send_to_sub {
    my ($existing_users) = @_;
    print Dumper($existing_users);
}
[download]