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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