It isn't really clear what you are trying to accomplish with unpack so it is difficult to tell what your problem is.

Under pack/unpack, 'A' only works with ASCII strings, so that is unlikely to be what you want. 'U' returns the ordinal of a utf-8 character. That may be useful to you, I can't tell from your question.

It seems like you are just trying to get the first 4 characters from the line, in which case, unpack is probably not the best way to go. You may be better of using substr

use warnings;
use strict;

my $infile  = 'D:/try_unicode/input.txt';  # or whatever
my $outfile = 'D:/try_unicode/output.doc';

open my $input,  '<:utf8', $infile  or die "Couldn't open file: $!";
open my $output, '>:utf8', $outfile or die "Couldn't open file: $!";

while (my $line = <$input>) {
    chomp $line;
    
    print $output 'The first four utf-8 chars from the line: ',
      pack( "U4", unpack( "U4", $line ) ),"\n";
      
    print $output 'Or, an easier way to get the same thing:  ',
      substr $line, 0, 4;
      
    print $output "\n", '-' x 79, "\n";
}
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$chinese=<INPUT>;
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that file contains the sequence of four chinese characters in a line.it is getting the four characters , i have problem in unpack operation.

Have you

use utf8;
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in your script ?

i tried the following code.but still problem
the input.txt contains 4 chinese characters in a line.

use utf8;
use Encode;
open INPUT,"<:utf8","D:\\try_unicode\\input.txt" or die "Couldn't open
+ file: $!";

close STDOUT;
open STDOUT ,">:utf8","D:\\try_unicode\\output.doc" or die "Couldn't o
+pen file: $!";

$unpacking=<INPUT>;
chomp($unpacking);
print "\nunpacking1   :" ,decode("utf8",unpack("x4A4",$unpacking)),"\n
+";
print "\nunpacking2   :" ,unpack("U4A4",$unpacking),"\n";

close INPUT;
close STDOUT;
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i used U4 still its giving some sequence of numbers.
i trien to x4 to null byte the four chinese characters its not working .
i tried using A4 also its not working.

I don't understand what you're trying to do with "unpack" here.