OK, I'll have a stab at how I would do your code-
some shortcuts for simplicity sakes, no error checking

my $day = ($base_vars{'day'} > $q->param('arrday')) ? 
  $base_vars{'day'} : $q->param('arrday');

my $days_select = '<select><option></option>';
for (1..31) 
{
  my $s = ($_ == $day) ? ' selected' : '';
  $days_select .= qq{<option$s></option>\n};
}
[download]

Untested. I hate calling a function (method) inside the loop if I can avoid it.

Tiago