%vars = (
   FTPPath      => '/my/ftp/path',
   TransferMode => 'ASCII',
);

s/(\$(\w+|{(\$|\w+)}))/
   my $var = defined($3) ? $3 : $2;
   $var eq '$' ? '$' : (defined($vars{$var}) ? $vars{$var} : $1);
/eg;