perl -e '
  $f = "\$foo \$(bar) \n";
  $f =~ s/\$ (?: (\w+) | \((\w+)\) )/<$^N>/xg;
  print $f;'

##</code><code>##

perl -e '
  my $f = "\$foo \$(bar)";

  our $val;       # must use package global
                  # for temporization in regex

  $f =~ s{\$                        # the dollar
    (?:                             # outer altinator
      (\w+)   (?{ $val = $^N })     # match and then store
    | \((\w+) (?{ $val = $^N }) \)  # or match 2 and store
    )                               # close outer
  }{<$val>}xg; print "$f\n"'