my $token = "__UNIQUE_TOKEN__";
  $token .= "_" while $str =~ /$token/;
  chomp (my $new_str = eval qq(<<"$token"\n$str\n$token));

##</code><code>##

   my %vars = ( var => "foo" );
   $str = 'abc $var def';
   (my $new_str = $str) =~ /\$(\w+)/$vars{$1}/g;