while ( ($k,$v) = each %set2 ){
    $set1{$k} = undef ; # or perhaps more useful, ++
}

# or to merge two and leave them both intact.
my %merged = ()
foreach $hashref ( \%set1, \%set2 ) {
    while (($k, $v) = each %$hashref) {
        $merged{$k} = $v;
    }
}