Very mysterious. Try and get a closer look at the string:

print "$_ " foreach(unpack('C*',$client_hostname));
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I notice here that ascii 23 (octal 27), the "end of transmission block" character prints as a space, but is not removed by \s.

ps. 'g' is superfluous in that regexp.

       sysread($rh,$buf,4096) || syslog ( "crit", "$prog : $!" );
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You do realize that if sysread returns 0 it is not a critical error and $! will not be set.

       if($buf) {
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You do realize that the string "0" is also false?

           my $client_hostname=unpack('a*', $buf);
           $client_hostname =~ s/\s+$//g;
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If you change the unpack format from 'a*' to 'A*' you won't need the substitution on the next line and it will also remove "\0" characters.

That's it! Thanks!!!

I think you getting confused with the trailing spaces and the whitespaces. Trailing spaces are the spaces that comes at the end of the last character. In your case "]" is the last character. Please look at the code below:

#!/usr/bin/perl

use strict;
use warnings;

my $str1 = "The tailing spaces example 1     "; ## spaces after 1
$str1 =~ s/\s+$//g;
my $str2 = "The tailing spaces example 2"; 
$str2 =~ s/\s+$//g;

print "Output 1:",$str1,"\n"; ## this removes the spaces after 1
print "Output 2:",$str1,"\n"; ## does nothing
print "Output 3:",$str1.$str2,"\n"; ## removes spaces and concatenates
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OUTPUT:

Output 1:The tailing spaces example 1
Output 2:The tailing spaces example 2
Output 3:The tailing spaces example 1The tailing spaces example 2
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Therefore, I would like to know what are you trying to do. If you want to remove the white spaces then you have to remove "$" from you substitution command i.e.:

$str1 =~ s/\s+//g;
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the the output will be as follows:

Output 1:Thetailingspacesexample1
Output 2:Thetailingspacesexample2
Output 3:Thetailingspacesexample1Thetailingspacesexample2
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Look more closely. The variable he's trying to remove a visible space from -- which apparently is only printing as a space but is some other invisible-at-the-terminal, non-space character -- is being interpolated into another string for printing. The string into which that variable is being interpolated is the one with the square brackets.