open(FILE,"<index.html");
my $html = <FILE>;
close(FILE);
$html =~ /<\/body>/;
my $top = $`;
my $bottom = $';

$top .= "<p>追加する文字列</p>\n";
$top .= $bottom;

open(FILE,">index.html");
print <FILE> $top;
close(FILE);

exit;

This is not confident.
Please point it out when the mistake is found.