HTML filter on Template Toolkit. perl

Question

Can someone tell me why in others code I saw expression like: [% variable | html | html %]. What is the reason to use two html filters? For example in perl catalyst controller:

sub index :Path Args(0) {
    my($self, $c) = @_;

    $c->stash->{template} = 'index.tt';
    $c->stash->{variable} = 'hello';
}

Index.tt:
<p> [% variable | html | html %] world</p>