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

perl -e '
  my $f = "\$foo \$(bar)";
...
    | \((\w+) (?{ $val = $^N }) \)  # or match 2 and store
    )                               # close outer
  }{<$val>}xg; print "$f\n"'