my $var1 = $cgi->param('var1');
my $var2 = $cgi->param('var2');
my $var3 = $cgi->param('var3');

my $h = { 'var1' => $var1, 'var2' => $var2, 'var3' => $var3 };
my @list = qw(var1 var3);

foreach (@list) { if (!exists $$h{$_}) { &complain; } }