use File::Slurp;
# (...)
my $current_string = "";
my $current_size = 0;
for my $file (@html_files) {
    my $new_file_string = read_file($file); # File::Slurp function
    my $len = length $new_file_string;
    if ($current_size + $len > $size_limit) {
        convert_to_pdf($current_string);
        $current_string = $new_file_string;
        $current_size = $len;
    } else {
        $current_string .= $new_file_string;
        $current_size += $len;
    }
}
convert_to_pdf($current_string) if $current_string;