#!/usr/local/bin/perl -w

use strict;

my %hsh = (a=>1,c=>3,d=>undef);

my %hsh1 = myfunc(%hsh);

while (my ($key, $value) = each %hsh1) {
 print "$key=$value\n";
}

sub myfunc {
 my %hsh = @_;
 my %dflt_values = (a=>0,b=>0,c=>0,d=>0);
 $hsh{$_} = $dflt_values{$_} for grep { not defined $hsh{$_} } keys %dflt_values;
 %hsh;
}