A related question that may help shed some light is
"why is 'my' documented in the 'perlfunc' man page?"
The answer would seem to be that 'my' has both compile-time
and run-time effects. At compile time, the Perl interpreter
decides the scope of the variable based on lexical scoping
rules. At run time, Perl allocates memory for the variable
and starts tracking references to that memory. This latter step happens each time the line is excuted.
This is why a mod_perl script should never use 'my' to
declare a 'global' variable. At compile time, any block
which refers to the variable will get pointed at the first
instantiation. On the second invocation, when the 'my' line
is reached, a new variable is created but all the other
blocks are still pointing at the old one.