needs to be split into two smaller strings

Your code creates more than two smaller strings/files. What do you want? Two equal-sized strings/files or strings/files with a maximum length?

strings [...] to be passed to a PDF converter

Blindly splitting HTML at arbitary offsets will most likely create invalid HTML. Depending on how it is rendered, you will lose some information. In the worst case, the converter may simply refuse to render it at all.

Back to your posted problem, assuming strings/files with a size limit, maybe more than two parts: Have you tried substr? Something like this (untested):

if (length($html[$i]) > 1000000) {
    my $z=0;
    while ($html[$i] ne '') {
        open (my $file, '>', "giant_${i}_$z.htm") or die $!;
        print $file substr($html[$i],0,500000,'');
        close $file;
        push(@lists, "giant_${i}_$z.htm");
        $z++;
    }
} else {
    # ...
[download]

Note that the above code is destructive due to using a four-argument substr, i.e. $html[$i] will be empty after the code has run. Non-desstructive code can use the $z counter, like this:

if (length($html[$i]) > 1000000) {
    my $z=0;
    while ($html[$i] ne '') {
        open (my $file, '>', "giant_${i}_$z.htm") or die $!;
        print $file substr($html[$i],$z*500000,($z+1)*500000);
        close $file;
        push(@lists, "giant_${i}_$z.htm");
        $z++;
    }
} else {
    # ...
[download]

Alexander