my %hash = (
   username => $form_username,
   password => $form_password,
   name     => $form_name,
);   

# was trying to do this with map ... but time is short ;)
for (keys %hash) {
   delete $hash{$_} unless $hash{$_};
}

my $sql = 'select from table where ' .
   join(' and ', map { qq|$_ = "$hash{$_}"| } keys %hash);