This should work for you

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

use utf8;
my $filename = "thaikjv-fixed.txt";

open my $THAI, $filename or die $!\n";
binmode ($THAI, ":utf8");

while (<$THAI>) {
    chomp $_;
    
    if ($_ =~ m/^@(...\d\d\d)/){
        my $r = "$1.html";
        open my $OUT, '>', $r or die $!\n";
        binmode ($OUT, ":utf8");
        print $OUT $line;
        close($OUT);
    }
}
close($THAI);
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