You could do the regexp thing: $string =~ s/month=\d+/month=$newmonth/g;, or if the code you are working on is calendarview.pl, and you already have a CGI.pm object you are working with, I find this is a really easy way to get slightly modified URLs:

my $cgi = new CGI;
# .... bunch of calendarview.pl code here ....

# need to get a url that takes us back to new years
my $newyears = new CGI($cgi);
$newyears->param('month',1);
$newyears->param('day',1);
my $newyears_url = $newyears->self_url;
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Generally I just abstract this into a little subroutine:

sub new_self_url {
    my %params = @_;
    my $new = new CGI($cgi);
    foreach(keys %params) {
        $new->param($_,$params{$_});
    }
    return $new->self_url;
}

# now you can get urls back to your script with different
# dates like this:
my $newyears = new_self_url(month => 1, day => 1);
my $christmas = new_self_url(month => 12, day => 25);
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