There is a newline character at the end of each line after you press ENTER.
To remove this unwanted character, Perl has the chomp function which does the trick.

my $s="";
while(chomp($s.=<STDIN>)){print "$s\n"}
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1. Spew warnings; spit errors.... or what?
2. and what OS is this running under?
3. and HOW are you entering the input -- as a set of words on the CLI invoking the code, properly quoted; as individual words once the code is running; by telephathic instantiation?

BTW, your code works as expected, when used to process input in the manner you've specified with <STDIN>... which is not the same way as it would work were you trying to process @ARGV.

Sorry I saw the output wrong. Actually It seems to work fine. And next time I will write more detailed question thanks!

Sorry I saw the output wrong. Actually It seems to work fine.

I find it highly unlikely that your code works, considering it's very likely to go into an infinite loop...

I agree with ww, it does work as expected, you are concatenating the newline character. trizen suggests a solution to remove that character. However I think the issue here is how to exit the while loop. If you assign and concatenate you attach the exit keystroke onto the variable you are processing. Mixing your control with your data.

#!/usr/bin/perl -wl
use strict;
my $s;
my $chomp = 1; # actually I do want to remove the newline;

 while(<STDIN>){
    chomp if $chomp;
  unless( /^q$/i ){ # exit from inputting is 'q' or 'Q'
     $s .= $_;      # concatenate
     print $s;      
  }else{ last }    
 }

print $s;           

exit 0;
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nice trick though, hadn't thought of using operators other than simple assignment

As soon as $s contains a "true" value, your loop will not terminate because the loop condition will always be true.

trizen's suggestion doesn't do that because chomp returns the number of characters removed, which will be zero when end-of-file is reached on STDIN - but it won't terminate on a blank line, since that still has a newline character. Personally, I might have written it like this:

while(<STDIN>) { chomp; $s.=$_; print "$s\n"; }
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However, putting the reading of STDIN in the condition of the while makes me suspect that you want to terminate the loop when an empty line is input. If that's what you want, here's one way to do that:

while(<STDIN>) { chomp; last unless length; $s.=$_; print "$s\n"; }
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Also, often, using <> instead of <STDIN> can be helpful because it gives you the power of Perl's magic ARGV (see I/O Operators). Unless you explicitly want to read from STDIN, I'd suggest using <>.