If no value is assigned to a scalar variable, it contains undef.
The example in the book doesn't want to show you what undef is, though. You should experiment with assigning various values to the variables and running the comparison, e.g.
my $alice = 'Alice'; my $bob = 'Bob'; say "Catastrophic crypto fail!" if $alice eq $bob;
my $alice = 'bob'; my $bob = reverse $alice; say "Catastrophic crypto fail!" if $alice eq $bob;