my %hash_defaults = ('a'=>0 , 'e'=>0 , 'i'=>0 , 'o'=>0 , 'u'=> 0,);
my %hash = %hash_defaults;

...

# Reinitialize
%hash = %hash_defaults;

##</code><code>##

while (...) {
    my %hash = ('a'=>0 , 'e'=>0 , 'i'=>0 , 'o'=>0 , 'u'=> 0,);
    ...
};

##</code><code>##

use vars '%hash';

%hash = ('a'=>0 , 'e'=>0 , 'i'=>0 , 'o'=>0 , 'u'=> 0,);
...
do {
    local %hash = %hash;
    ... make some changes
};
... changes are forgotten