my $hash{key} = ...; # is illegal
local $hash{$key} = ...; # is legal
#on the other hand
my $x = 1;
{
  my $x = 2; # is legal, a new variable was created, outer $x is not visible
}
#while
my $x = 1;
{
  local $x = 2; # is illegal. Can't localize a lexical variable