I'd suggest to use while(<FH>) instead of foreach(<FH>). (while loop reads directly from file, for(each) loop slurps the entire file into memory first - really bad for huge files, but ok for small ones).

Maybe something like this:

use strict;
use warnings;

open my $thai_fh, '<:utf8', "thaikjv-fixed.txt" or die $!;

while(<$thai_fh>){
    if (/^@(...\d\d\d)/) {
        open my $out_fh, '>:utf8', "$1.html" or die $!;
        print {$out_fh} $_;
    }
}
[download]

/code