Is this possible?

No.

From the docs

...it enforces an important divide - design and programming.

The idea behind HTML::Template is that what you are trying to do is to put 'programming' in your template.

Why can't you put that regex in your param, something like

my $flag;
if (my_var =~ /some string/){
  $flag++;
}

my $param = {
  flag => $flag,
};
[download]

<TMPL_IF flag>
[download]

imo, this restriction is a good thing. It forces you to separate your logic from your presentation, and, I think, both your code and your templates are better for it. Another benifit is that H::T is a fast, easy to use, lightweight solution for most needs.

Perhaps consider something like Template::Toolkit if H::T is too restrictive for your needs.

updated: fixed link