Problems like this can be avoided with the strict pragma. See Use strict warnings and diagnostics or die for a more thorough discussion.

my $result = data();

foreach my $res (@$result) {

        my $name = $res->{'username'};
        my $city = $res->{'city'};
        print "User Name = $name - City: $city\n";
    }

my @all_data = ();
sub data {
...
db code stuff
...
 my %no_name_found;
 if(@$sql) {
     
   for(my $i = 0; $i < @$sql; $i++) {


     my $user_name   = $sql->[$i]{'username'}  || '';
     my $city      = $sql->[$i]{'city'}  || '';

     # verify user name, this name will come back from this sub check_
+name
     my $no_name = check_name (
                               user => $user_name,
                               );
     
    #Get a new hash for the data
    my %data;
    $data{username} = $user_name;
    $data{city}      = $city;
    push @all_data, \%data;
   

   }
  
  
 }
 
 return \@all_data;

}
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