$ ./my_with_if.pl
$param is undef
$my_var after declaration is "default"
    (as expected)
$my_var after '//=' is "default"

$param is "call 1"
$my_var after declaration is undef
    (as expected)
$my_var after '//=' is "call 1"

$param is "call 2"
$my_var after declaration is "call 1"
    (unexpected to me!)
$my_var after '//=' is "call 1"

$param is "call 3"
$my_var after declaration is "call 1"
    (unexpected to me!)
$my_var after '//=' is "call 1"