Maybe it's best to use an existing implementation as the base interface. I would propose Template Toolkit because it seems to have the most comprehensive API of all templating systems. The "TPD" for Template Toolkit would virtually do nothing other than fowarding method calls to TT directly. Other template systems would need a small wrapper module to translate the TT-like method calls. Generally only a subset of the TT functionality would be supported, but this should be in most cases sufficient.

Here's a quick sketch how this could look like:

#!/usr/bin/perl -w

use strict;

{
    package MetaTemplate::TT;
    use base qw(Template);
}

{
    package MetaTemplate::HTML_Template;
    use HTML::Template;
    sub new {
    my $class = shift;
    bless {}, $class;
    }
    sub process {
    my($self, $input, $vars, $output) = @_;
    my @ht_args;
    if (ref $input eq 'SCALAR') {
        @ht_args = (scalarref => $input);
    } else {
        @ht_args = (filename => $input);
    }
    my $ht = HTML::Template->new(@ht_args);
    if ($vars) {
        while(my($k,$v) = each %$vars) {
        $ht->param($k => $v);
        }
    }
    my $o = $ht->output;
    if (ref $output eq 'SCALAR') {
        $$output = $o;
    } elsif (defined $output) {
        open(my $OUT, "> $output") or die $!;
        print $OUT $o;
    } else {
        print $o;
    }
    }
}

{
    package MetaTemplate::Text_ScriptTemplate;
    use Text::ScriptTemplate;
    sub new {
    my $class = shift;
    bless {}, $class;
    }
    sub process {
    my($self, $input, $vars, $output) = @_;
    my $tmpl = Text::ScriptTemplate->new;
    if (ref $input eq 'SCALAR') {
        $tmpl->pack($$input);
    } else {
        $tmpl->load($input);
    }
    if ($vars) {
        $tmpl->setq(%$vars);
    }
    my $o = $tmpl->fill;
    if (ref $output eq 'SCALAR') {
        $$output = $o;
    } elsif (defined $output) {
        open(my $OUT, "> $output") or die $!;
        print $OUT $o;
    } else {
        print $o;
    }
    }
}


my $t1 = MetaTemplate::TT->new;
$t1->process(\<<EOF, { answer => 42 });
The answer is [% answer %].
EOF

my $t2 = MetaTemplate::HTML_Template->new;
$t2->process(\<<EOF, { answer => 42 });
The answer is <TMPL_VAR NAME=answer>.
EOF

my $t3 = MetaTemplate::Text_ScriptTemplate->new;
$t3->process(\<<'EOF', { answer => 42 });
The answer is <%=$answer%>.
EOF
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