Moved reading the directory to the outside of the loop and some other cleanup:

#!/usr/bin/perl
 
use warnings;
use strict;

use File::Spec::Functions qw( catfile );

my $src_dir = "C:\\ROOT_DIR\\test1\\";
my $dst_dir = "C:\\ROOT_DIR\\test2\\";

opendir(my $dh, $srcdir)
   or die "Can't open dir $srcdir: $!\n";

while (defined(my $file = readdir($dh))) {
    next if /^\.\.?\z/;

    my $src_file = catfile($src_dir, $file);
    my $dst_file = catfile($dst_dir, $file);
 
    open(my $src_fh, "<:raw:perlio:encoding(UTF-8)", $src_file)
       or die("Can't open \"$src_file\": $!\n");

    open(my $dst_fh, ">:raw:perlio:encoding(UTF-16)", $dst_file)
       or die("Can't open \"$dst_file\": $!\n");
  
    print $dst_fh $_ while <$src_fh>;

    unlink($src_fh);
}

print "done.\n";
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