What hippo said++.

The /$_(\h\.)? fragment still does not compile. If I "fix" it to /$_(\h\.)?/ it doesn't make sense because $_ is the line/record read from the input file (story.rst in the example), so the equivalent expression is
    $_ =~ /$_(\h\.)?/
The (\h\.)? tacked on at the end is optional and therefore kinda meaningless, so the expression more or less (!) reduces to
    $_ eq $_
unless $_ contains regex metacharacters, in which case all bets are off.

A possible way forward would be for you to post a small example of the typical content of the story.rst file and of the output you expect from processing the given content (update: see Short, Self-Contained, Correct Example for this general technique). The best way to do this is usually as a reply to your OP. If you want to update the OP, please see How do I change/delete my post? as suggested by hippo.

... is astray to s.t. ...

I still don't understand what this means.

N.B: The statement
    print if $seen{ $string }++
will print every line after the line in which $string (however $string is derived) is first found. Is this what you intend?