#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11158577
use warnings;

for ( split /\n/, <<END )
#foo bli bla
bli #foo bar baz
#more than #one is a #string
more than #one is a #string
foo bli bla
# does not match #
END
  {
  /(?|#(\S+)|()\z)/ and print "found '$1' in '$_'\n";
  }