for my $key (my @temp = keys %my_hash) {
  my $value = $my_hash{$key}    # and no need to change the code below
  .
  . # some processing on the element
  .
}

##</code><code>##

my %new;
foreach my $key (keys %old}
{
      if(....)
      {      
             $new{$key} = $old{$key};
             # do further processing.
      }
      else
      {
             print STDERR "dropped key: $key\n";
      }
}