# read through big string with changed end-of-line :)
# untested

my @bigHTML;
do
  {
  local $/ = '</html>';  # or  "</html>\n"  maybe
  open my $fh, '<', \$html[$i] or die "...";
  my $smallstring = '';
  while(<$fh>)
    {
    $smallstring .= $_;
    if( length $smallstring > 500_000 )
      {
      push @bigHTML, $smallstring;
      $smallstring = '';
      }
    }
  length $smallstring and push @bigHTML, $smallstring; # leftover
  };