What's the difference between dynamic and lexical (static) scoping? Between local() and my()?

local($x) saves away the old value of the global variable $x and assigns a new value for the duration of the subroutine which is visible in other functions called from that subroutine. This is done at run-time, so is called dynamic scoping. local() always affects global variables, also called package variables or dynamic variables.

my($x) creates a new variable that is only visible in the current subroutine. This is done at compile-time, so it is called lexical or static scoping. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables.

For instance:

    sub visible {
	print "var has value $var\n";
    }

    sub dynamic {
	local $var = 'local';	# new temporary value for the still-global
	visible();              #   variable called $var
    }

    sub lexical {
	my $var = 'private';    # new private variable, $var
	visible();              # (invisible outside of sub scope)
    }

    $var = 'global';

    visible();      		# prints global
    dynamic();      		# prints local
    lexical();      		# prints global

In summary, local() doesn't make what you think of as private, local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables.

See perlsub/"Private Variables via my()" and perlsub/"Temporary Values via local()" for excruciating details.