I really miss strict, warnings, taint mode, and RaiseError.

Interpolating variables into SQL statements usually triggers my "use placeholders" warning (Re: Counting rows Sqlite, Re^2: Massive Memory Leak). But in the only SQL statement you show here, you "just" interpolate a constant table name into the SQL statement. Not as bad as variable data from the network, but still, you should use DBI's quote_identifier() method. That won't even break when you change $DbUsers.

More:

• You use string eq to compare numbers. Better use ==. How to remember that.
• SELECT RegNum FROM Users ORDER BY RegNum DESC LIMIT 1 returns at most one value (LIMIT 1). That value is always the maximum value, due to ORDER BY RegNum DESC. Why don't you just SELECT MAX(RegNum) FROM Users? (MAX() in MySQL)
• Plus, the code that you posted does not even compile:

  X:\>perl 1199050.pl
  syntax error at 1199050.pl line 26, near ""$DBI::errstr":"
  syntax error at 1199050.pl line 34, near "}"
  Execution of 1199050.pl aborted due to compilation errors.
  
  X:\>
  [download]

  Better copy&paste from the actual source. Manual typing adds extra errors.

Alexander