Actually lexical variables can be localized.

use strict;
my %hash = qw(hello world);
print "$hash{hello}\n";
{
  local $hash{hello} = "WORLD";
  print "$hash{hello}\n";
}
print "$hash{hello}\n";
[download]

There is no technical reason why this isn't doable for the original hash as well. The only reason why it cannot currently be done is that it is considered a likely cause of confusion.