HTML::Template and its derivative HTML::Template::Pro don't support the kind of conditionals you need for that, just simple bools. So what you need to write is

            <select name="province_id" id="province">
                <option value="-1">--</option>
                <TMPL_LOOP NAME="provinceloop">
                    <option value="<TMPL_VAR NAME="id">"
                    <TMPL_IF is_selected>selected="true"</TMPL_IF>>   
+   
                <TMPL_VAR NAME="name"></option>
                </TMPL_LOOP>
            </select>
[download]

And in the the perl code populate is_selected with a true value for the option where id eq current_key.

    <tmpl_html_option province>
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    $htc->param(
        province => [ 'opt_2', # selected
            ['opt_1', 'option 1'],
            ['opt_2', 'option 2'],
        ],
    );
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    <option value="opt_1">option 1</option>
    <option value="opt_2" selected="selected">option 2</option>
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<TMPL_LOOP NAME="provinceloop">
  
  <option 
    value     = "<TMPL_VAR NAME = "id">"
    selected  = "<TMPL_VAR NAME = "current_key">
  >      
    <TMPL_VAR NAME="name">
  </option>
  
</TMPL_LOOP>
[download]

How do I post a question effectively?

It may not be Perl code, but it is a Perl question. The sample is HTML with HTML::Template mark up added. If you deal with HTML generation at all then you would do very well to take a look at the module.

Oh, and while we are disabusing notions: the language is Perl. The interpreter is perl. This is a common shibboleth in Perl speaking circles.

True laziness is hard work

Thank you - I've learned new (for me) word.