$matchtext = "http://www.foo.bar/somepage.pl?";
$substitutetext = "http://www.new.site/somenewpage?";
s/$matchtext/$substitutetext/;

##</code><code>##

$matchtext = "http://www.new.site/somenewpage.pl\?\?";
$substitutetext = "http://www.new.site/somenewpage.pl\?";
s/$matchtext/$substitutetext/;