$str = "calendarview.pl?loginid=102138&month=04&year=2003&student_id=&user_type=TUTOR&CalendarName=102138Academic&framename=top.index_main&session_number=618280744437303";

# first amputate the calendarview.pl? bit
$str =~ s/^[^\?]+\?//;

# use CGI.pm to parse the string by creating a CGI instance from it
use CGI;
$q = CGI->new($str);

# modify whatever you want, lets delete what you specified
$q->delete('CalendarName');
$q->delete('month');
$q->delete('year');

# and modify loginid just for laughs
$q->param( 'loginid', 'haxor' );

# add a new param if you want
$q->param('i am a new one', 'this is my value' );

# write out the new query string (default is to use ; not & so we do:)
$CGI::USE_PARAM_SEMICOLONS = 0;
print $q->query_string();

__DATA__
loginid=haxor&student_id=&user_type=TUTOR&framename=top.index_main&session_number=618280744437303&i%20am%20a%20new%20one=this%20is%20my%20value