sub convert {
    my ($template_name) = @_;
    my $template;

    {
       open my $fh, '<', $template_name
          or die("Unable to open template file \"$template_name\": $!\n");

       local $/;
       $template = <$fh>;
    }

   $template =~ s/~~\$(.+?)~~/... new syntax .../g;

    {
       open my $fh, '>', $template_name
          or die("Unable to overwrite template file \"$template_name\": $!\n");

       print $fh $template;
    }
}