Do you mean the template text is stored in a variable, not a filename ? Something like this using BLOCKS

#!/usr/bin/perl
use strict;
use Template;

my $tt = Template->new({
  BLOCKS => {
    tvar  => 'Text is [% text %]',
  },
});
 
my $vars = { text => 'Some text' };
$tt->process(\*DATA, $vars);

__DATA__
[% INCLUDE tvar %]
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