use warnings;

my @sites = (undef, "a", "b");
my $sitecount = 1;

while ($sites[$sitecount] ne undef) {
   $sitecount++;
}

Use of uninitialized value in string ne at t.pl line 6.
Use of uninitialized value in string ne at t.pl line 6.
Use of uninitialized value in string ne at t.pl line 6.
Use of uninitialized value in string ne at t.pl line 6.