the & prevents name clashes with perl function

& is not a no-op. It causes prototypes to be ignored.

I don't understand what local() does differently than my().

my should be used whenever possible, but it's not possible to use my on package variables.

>perl -e"my $_"
Can't use global $_ in "my" at -e line 1, at end of line
Execution of -e aborted due to compilation errors.
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$_ was made a legal name for a lexical in 5.10, and builtins will use the lexical $_ instead of the global $_ if there is one in scope, but not everyone is using 5.10 yet.

What does that * notation do

One reason my is preferred over local is that my creates a new variable, whereas local simply saves the variables current value (and restores it on scope exit). Any magic associated with the variable is unaffected by local.

You can localise the variable rather than its value by localising the glob that contains the variable. A glob is an associative array (like a hash) that contains the different type of variables ($name, @name, %name) associated with each name in the symbol table.

local *_ localises the whole glob, which means a new $_ will be created if one is needed.