Just a precedence issue. Add parens.

my %new2 =
hmap { $_[1]{nick} =~ /rbush|fred/
          ? ( $_[0] => { hmap { $_[0], $_[1] } %{$_[1]} } )
          : ()
     } %$h;
[download]

my $new2 =
hmap { $_[1]{nick} =~ /rbush|fred/
          ? ( $_[0] => hmap { $_[0], $_[1] } $_[1] )
          : ()
     } $h;
[download]