I am a terrible, horrible person

Foo.pm

use feature ':5.10.0';
package Foo;
use Text::Template;

# needs to be in scope for Text::Template's processing
my %data;

sub get_metadata {
  my ($tmpl) = @_;
  return Bar::get_metadata($tmpl);
}

sub fill {
  my ($tmpl, $user_input) = @_;

  my $meta = get_metadata($tmpl);
  for my $var_name (keys %{ $meta->{vars} || {} } ) {
    my $default = $meta->{vars}{default};
    $data{vars}{$var_name} = $user_input->{vars}{$var_name} // $default;
  }
  $data{select} = $user_input->{select};
  $data{where} = $user_input->{where};
  $data{where_mode} = $user_input->{where_mode};
  $data{limit} = $user_input->{limit};

  my $tt = Text::Template->new(TYPE => 'STRING', SOURCE => $tmpl );

  # strip out blank lines
  (my $new_hql = $tt->fill_in) =~ s/^\s*\n//gms;

  return $new_hql;
}



sub insert_var {
  my ($name, %opt) = @_;
  return $data{vars}{$name};
}

sub insert_select {
  my (%opt) = @_;
  return unless $data{select};
}

sub insert_where {
  my (%opt) = @_;
  return unless $data{where};
  my $mode = $data{where_mode} || 'AND';
  my $where_clause = join " $mode ", map { join ' ', @$_ } @{$data{where}};
  return unless $where_clause;
  return "WHERE $where_clause";
}

sub insert_limit {
  my (%opt) = @_;
  return unless $data{limit};
  return "LIMIT $data{limit}";
}

sub append_group_by {
  my (%opt) = @_;
  return unless $data{group_by};
  my $clause = ", " . join ", ", @{$data{group_by}};
  return $clause;
}


package Bar;

# same deal as %data, above
my %metadata;

sub get_metadata {
  my ($tmpl) = @_;
  my $tt = Text::Template->new(TYPE => 'STRING', SOURCE => $tmpl );
  $tt->fill_in;
  return \%metadata;
}

sub insert_var {
  my ($name, %opt) = @_;
  $metadata{vars}{$name} ||= \%opt;
  return;
}

sub insert_select {
  my (%opt) = @_;
  return;
}

sub insert_where {
  my (%opt) = @_;
  $metadata{where_columns} ||= $opt{columns};
  return;
}

sub insert_limit {
  my (%opt) = @_;
  $metadata{limit} ||= \%opt;
  return;
}

sub append_group_by {
  my (%opt) = @_;
  $metadata{group_columns} ||= $opt{columns};
  return;
}

1 && q{ I clearly have no shame }; # truth

sql_template.tql

SELECT g_vw.ds,
    g_vw.locale,
    round(((SUM(
      CASE
        WHEN g_vw.au_first_pageview_ts >= pg_vw.last_pageview_ts  THEN 1
        ELSE 0
      END
    ) / COUNT(*)) * 100.0), 3) as bounce_rate
FROM vw_discoverau_general g_vw
JOIN (
  SELECT fp.ds,
      fp.locale,
      fp.session_id,
      MAX( fp.request_finished ) as last_pageview_ts
  FROM f_pageviews_hist fp
  WHERE fp.ds ='{{ insert_var('view_date', default => '2012-08-22') }}'
  GROUP BY fp.ds, fp.locale, fp.session_id
  ) pg_vw
ON (
  g_vw.ds = '{{ insert_var('view_date') }}' AND g_vw.ds = pg_vw.ds AND
  g_vw.locale = pg_vw.locale AND g_vw.session_id = pg_vw.session_id )
{{
  insert_where( columns => [qw/g_vw.locale pg_vw.session_id/] );
}}
GROUP BY g_vw.ds, g_vw.locale
{{ append_group_by(columns => [qw/fp.session_id/]) }}
{{ insert_limit() }}