use strict;
my %vars = ("foo" => 1, "bar" => 2, "baz" =>3);
my %newvars = ("newfoo" => "A");

foreach my $var (keys %vars) {
  my $newvarname = "new" . $var;
  if (defined $newvars{$newvarname}) {
    $vars{$var} = $newvars{$newvarname};
  }
}