perl -w $your_script.pl
Name "main::ConfFile" used only once: possible typo at foo.pl line 16.
Name "main::AllDevi" used only once: possible typo at foo.pl line 8.
Name "main::STRM_ALLDevices" used only once: possible typo at foo.pl line 11.
print() on unopened filehandle STRM_ALLDevices at foo.pl line 11.
.
print() on unopened filehandle STRM_ALLDevices at foo.pl line 11.
...

##</code><code>##

use warnings;
use strict;
my $AllDevices='All_Devices.txt';
my $_AllConfDir = '.';

# Listing the devices

opendir (my $dir ,$_AllConfDir) or die "Error in opening dir $_AllConfDir\n";
open my $a, ">>",  $AllDevices
	or die "couldn't open file '$AllDevices for appending: $!";

while ( my $file = readdir($dir)) {
     print $a "$file\n";
     print "$file\n"; 
}
closedir $dir or warn $!;
close $a or warn $!;