while ( defined( my $first = <DATA> ) && defined( my $second = <DATA> ) ) {
    print spandiffstr( $first, $second );
    }

sub spandiffstr {
    my ( $old, $new ) = @_;
    my $span = '<span>';
    my $espan = '</span>';
    ( $old ^ $new ) =~ /^\0*/ and my $subscr = $+[0];
    my ( $rold, $rnew ) = map scalar reverse, $old, $new;
    ( $rold ^ $rnew ) =~ /^\0*/ and my $lsubscr = -$+[0];
    # got pertinent subscripts, now insert span tags
    substr $old, $lsubscr, 0, $espan;
    substr $new, $lsubscr, 0, $espan;
    substr $old, $subscr, 0, $span;
    substr $new, $subscr, 0, $span;
    return $old, $new;
    }