A bit more on this.

Files exists independently of directory entries in unix. Think of them as being referenced-counted. They exist as long as they are referenced by a directory entry or by a file handle.

This means you can have "anonymous files". perl -i uses this (which is why it doesn't work on Windows without an extension). File::Temp creates these by default.

ls -l even shows how many times a file is referenced by a directory entry (second column). For plain files, it's usually one. For directories, it should be more than one.

$ ls -ld . .. .bash* bin usr
drwxr-xr-x 10 ikegami ikegami 4096 Feb 19 23:53 ./
drwxr-xr-x  3 root    root    4096 Dec 10 21:14 ../
-rw-------  1 ikegami ikegami  163 Jan  3 02:07 .bash_logout
-rw-------  1 ikegami ikegami 2318 Jan  3 02:07 .bashrc
-rw-------  1 ikegami ikegami    0 Jan  3 02:07 .bashrc_extra
drwx------  2 ikegami ikegami 4096 Feb 14 12:13 bin/
drwx------  3 ikegami ikegami 4096 Jan 24 15:39 usr/
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See how .. is referenced by three directory entries? Those are /home, /home/. and /home/ikegami/..

See how . is referenced by ten directory entries? Those are /home/ikegami, /home/ikegami/., /home/ikegami/bin/.., /home/ikegami/usr/.., etc.

Probably that file and inode still exist without association to a dir, but how would I recreate it into the dir?

The directory entry is gone, yes. Writing to a file doesn't create directory entries.

I didn't mention glob at all. Did you try the steps I described? The file is only gone once no directory and no file handles reference it. Also, see the post I just made in parallel to yours.

$ perl -Mv5.10 -e'
   open my $o_fh, ">", "a" or die $!;
   open my $i_fh, "<", "a" or die $!;
   unlink "a" or die $!;
   print $o_fh "abc\n" or die $!;
   $o_fh->flush();
   chomp( my $line = <$i_fh> );
   say "<$line>";
'
<abc>
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> See the post I just made in parallel to yours.

I know about inodes

But the question remains:

How can I revive/move an "invisible" file into the directory?

link doesn't seem to accept FHs.

use strict;
use warnings;
use Data::Dump qw/ddx/;
use autodie;



my $dir  = "/tmp";
my $file = "$dir/tst_unlink";

open my $fh, ">>", $file;

print $fh "ENTRY ".__LINE__;

ddx <$file*>;

unlink $file;

print $fh "ENTRY ".__LINE__;

ddx <$file*>;

eval { link $fh, "${file}_new"; } or warn $@;

eval { link $file, "${file}_new"; } or warn $@;


ddx <$file*>;

__DATA__
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OUTPUT:

# open_unlink.pl:16: "/tmp/tst_unlink"
# open_unlink.pl:22: ()
Can't link($fh, '/tmp/tst_unlink_new'): Datei oder Verzeichnis nicht g
+efunden at ./open_unlink.pl line 24
Can't link('/tmp/tst_unlink', '/tmp/tst_unlink_new'): Datei oder Verze
+ichnis nicht gefunden at ./open_unlink.pl line 26
# open_unlink.pl:29: ()
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Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

PS: FWIW: I was guessing, I can't run a full diagnosis for every fuzzy bug question, even after reading it thrice.