In the content, {{$NEXT}}
is being interpreted as a template block and, as you say, wants to fill itself in as the contents of the missing $NEXT
.
Instead, try:
content = {{'{{$NEXT}}'}}
Example program:
use 5.14.0;
use Text::Template 'fill_in_string';
my $result = fill_in_string(
q<{{'{{$NEXT}}'}}>,
DELIMITERS => [ '{{', '}}' ],
BROKEN => sub { die },
);
say $result;