as CountZero says -- you've got a situation where the you're referencing something that contains only a value and not a hash ref:

Using your Data structure we walk trough your code and substitute in the values to see the problem.

$My_Value = 'SuperUser'; # assumption for this exercise

foreach $item (keys %Myhash){ 
# $item = 'SuperUser' first pass through
    if ($item eq $My_Value)    {
    foreach $iteminitem (sort(keys %{$Myhash{'SuperUser'}})){
# $iteminitem = 'Username' first pass through
            foreach $iteminitem1 (sort(keys %{$Myhash{'SuperUser'}{'Us
+ername'}})){
## Here's the problem: $Myhash{'SuperUser'}{'Username'} = 2
        if ($iteminitem1 eq 'name' &&  $user_number == $Myhash{$item}{
+$iteminitem}{$iteminitem1}) {
        my $User = $iteminitem; 
              }
            }
          }
        }
     }
[download]

What you probably want to do is check that you have a hash ref in $iteminitem

wrap the line 209 loop in a check

if (ref $iteminitem eq 'HASH') {
    foreach $iteminitem1 (sort(keys %{$Myhash{$item}{$iteminitem}})){
....
    }
}
[download]

Or perhaps it's time to refactor the code a bit, but without further context that's difficult to say