Recommend if you must, but please remember I'm looking for advice on semantics, not implementation.

Mine existing implementations for semantics? Ha:D

Text::Xslate::Syntax::Kolon is speedyfast, see Text::Xslate::Manual::FAQ, Xslate - Scalable template engine for Perl5

I like Mojo::Template/TT2 deal where its MARKERattribute varname attributeMARKER as in

[%-  trim_prev_trail_whitespace -%]
[%- trim_prev_whitespace %]
[% trim_trail_whitespace | html -%]
<%# comment line then trailing whitespace trimmed =%>
<%== Perl expression, replaced with XML escaped result %>
<: $textarea | html :>
[download]