Your success with append mode might be Perl version/system dependent. On my macbook, I still get the black hole when I reopen in append mode.

I mean

     open($fh1, '>', \ substr($s1, length($s1)) )
       or die "Can't restore fh1";
[download]

replace '>' with '>>' and \ substr($s1, length($s1)) with \$s1 and you'll get

     open($fh1, '>>', \$s1)
       or die "Can't restore fh1";
[download]

I'm also not comfortable with the idea that "one shouldn't expect" a file handle to work with dup. As far as I know, Perl doesn't give me a way to tell if a file handle was created on a string.There is no way to gracefully degrade or warn the user if they hand my subroutine a file handle and it just happens to be backed with a string.

Well I'm not saying that open is being reasonable here. IMO, yes, it should work, or at least warn if it doesn't. That being said, there is a way to tell the difference

Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. If there is no real file descriptor at the OS level, as can happen with filehandles connected to memory objects via "open" with a reference for the third argument, -1 is returned. - fileno