Thanks Anonymous Monk for those links. They were helpful, but still I am unable to solve my problem. I tried as follows:

RewriteRule ^http\:\/\/localhost\/sample\/prog\.pl\?q\=param\:A\;pageo
+ffset\=10$ http://localhost/sample/prog.pl?q=param:B;pageoffset=20
RewriteRule ^prog\.pl\?q\=param\:A\;pageoffset\=10$ prog.pl?q=param:B;
+pageoffset=20
[download]

After this, when I open page 'http://localhost/sample/prog.pl?q=param:A;pageoffset=10' in the browser, I get the content of this same page and not the page that is intended/directed (i.e. http://localhost/sample/prog.pl?q=param:B;pageoffset=20). Any suggestions? Thanks again.