$line='Sampel Text <a href="somefilename(x)withanumberedhash.htm#666">';
$somethingelse='ok';
$var = '<a href="somefilename(x)withanumberedhash.htm#666">';

$var=quotemeta($var);
$line=~s@$var@$somethingelse@;

##</code><code>##

$line=~s@\Q$var\E@$somethingelse@;