Except that will give you warnings if the page parameter is actually non-existent.

You want to test definedness as well as emptiness.

my $page = $cgi->param( "page" );
unless( defined $page and $page ne "" ){ $page = "login"; }
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Of course that's pretty clusmy and contains lots of unnecessary punctuation. Let's use a statement modifier instead:

my $page = $cgi->param("page");
$page = "login" unless defined $page and $page ne "";
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