I'm just not seeing how to implement the alternatives in my specific case

Well, its difficult to know, since you don't show why you need a bunch of $newfoo variables, but the approach with a hash might be to have two hashes, one for the original vars, and one for the new vars; so something like

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};
  }
}
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Dave.