Test oneliner without double escaping dollar sign

Fix Perl-Toolchain-Gang#355

Double escaping '$' for Makefile usage is fine, but
we should not use such a syntax while testing and trying
to run the oneliner output.

This change is 'unescaping' the '$$' to '$' so we can perform
some extra checks while using oneliner.

Otherwise as shown in this example a simple 'my $foo' test will fail.

t/oneliner.t

@@ -8,7 +8,7 @@ chdir 't';
 
 use Config;
 use MakeMaker::Test::Utils;
-use Test::More tests => 16;
+use Test::More tests => 17;
 use File::Spec;
 
 my $TB = Test::More->builder;
@@ -24,6 +24,7 @@ isa_ok($mm, 'ExtUtils::MM_Any');
 sub try_oneliner {
     my($code, $switches, $expect, $name) = @_;
     my $cmd = $mm->oneliner($code, $switches);
+    $cmd =~ s{\$\$}{\$}g; # unescape Makefile syntax
     $cmd =~ s{\$\(ABSPERLRUN\)}{$perl};
 
     # VMS likes to put newlines at the end of commands if there isn't
@@ -37,7 +38,8 @@ sub try_oneliner {
 try_oneliner(q{print "foo'o", ' bar"ar'}, [],  q{foo'o bar"ar},  'quotes');
 
 # How about dollar signs?
-try_oneliner(q{$PATH = 'foo'; print $PATH},[], q{foo},   'dollar signs' );
+try_oneliner(q{my $PATH = 'foo'; print $PATH},[], q{foo},   'dollar signs' );
+try_oneliner(q{my %h = (1, 2); print $h{1}},[], q{2},   '%h and $h' );
 
 # switches?
 try_oneliner(q{print 'foo'}, ['-l'],           "foo\n",       'switches' );