Yes, Perl variables are backed by internal data structures (called SVs) that include a flag to indicate whether a value is defined. So you're right — Perl keeps track of whether a variable is defined or not. When you use an undefined variable, Perl tries to handle it gracefully: in numeric context it becomes 0, and in string context it becomes an empty string "". But this can easily hide bugs if you're not careful. To catch such mistakes, always use use strict; use warnings; and check with defined($var) when needed.