I like to use simpler tags for variables in templates so I use the filter to turn %~foo~% into <TMPL_VAR NAME="foo"> like so:

use HTML::Template;
my $tmpl = HTML::Template->new(
    scalarref         => \do { local $/; <DATA> },
    die_on_bad_params => 0,
    loop_context_vars => 1,
    filter            => sub {
        my $s=shift;
        $$s=~s/%~([^~]+)~%/<TMPL_VAR NAME="$1">/g;
     },
);
$tmpl->param( userid   => 99999 );
$tmpl->param( username => 'foo' );
print $tmpl->output;
__DATA__
<a href="%~userid~%">%~username~%</a>
is easier to edit than
<a href="<TMPL_VAR NAME="userid">"><TMPL_VAR NAME="username"></a>
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