AutoFilter.pm

use strict;
use warnings;

package Template::AutoFilter;

use base 'Template';

use lib '..';
use Template::AutoFilter::Parser;

sub new {
    my $class = shift;

    my $params = defined($_[0]) && ref($_[0]) eq 'HASH' ? shift : {@_};
    $params->{FILTERS}{none} ||= sub { $_[0] };

    $params->{PARSER} ||= Template::AutoFilter::Parser->new( $params );

    return $class->SUPER::new( $params );
}

1;

autofilter.t

#!/usr/bin/perl

use strict;
use warnings;

package autofilter;

use lib 'lib';
use lib '../lib';

use Test::Most;

run_tests();
done_testing;

exit;

sub tests {(
    {
        name => 'plain text remains unfiltered',
        tmpl => '<a>',
        expect => '<a>',
    },
    {
        name => 'excluded tokens remain unfiltered',
        tmpl => '[% test | none %]',
        expect => '<a>',
    },
    {
        name => 'unfiltered tokens get filtered',
        tmpl => '[% test %]',
        expect => '&lt;a&gt;',
    },
    {
        name => 'specifically filtered tokens get filtered',
        tmpl => '[% test | html %]',
        expect => '&lt;a&gt;',
    },
    {
        name => 'other filters are applied without the autofilter',
        tmpl => '[% test | upper %]',
        expect => '<A>',
    },
)}

sub run_tests {
    use_ok "Template::AutoFilter";

    run_test($_) for tests();

    return;
}

sub run_test {
    my ( $test ) = @_;
    $test->{params} ||= {};

    my $tt = Template::AutoFilter->new( $test->{params} );
    my $out;
    my $res = $tt->process( \$test->{tmpl}, { test => '<a>' }, \$out );

    subtest $test->{name} => sub {
        cmp_deeply( [ $tt->error, $res ], [ '', 1 ], 'no template errors' );

        is( $out, $test->{expect}, 'output is correct' );
    };

    return;
}

Parser.pm

use strict;
use warnings;

package Template::AutoFilter::Parser;

use base 'Template::Parser';

sub new {
    my ( $class, $params ) = @_;

    my $self = $class->SUPER::new( $params );
    $self->{AUTO_FILTER} = $params->{AUTO_FILTER} || 'html';

    return $self;
}

sub split_text {
    my ( $self, @args ) = @_;
    my $tokens = $self->SUPER::split_text( @args ) or return;

    for my $token ( @{$tokens} ) {
        next if !ref $token;

        my %fields = @{$token->[2]};
        next if $fields{FILTER};

        push @{$token->[2]}, qw( FILTER | IDENT ), $self->{AUTO_FILTER};
    }

    return $tokens;
}

1;