perl -wle'
use Data::Dumper;
my $data = {
  q/foo@t-online.example/ => 1,
  q/foo@-online.example/ => 1,
};

my $domain = "-online.example";
foreach my $email (keys %$data) {
  if ($email =~ m/\@$domain$/) {
    delete $data->{$email};
  }
}
print Dumper $data;'
$VAR1 = {
          'foo@t-online.example' => 1
        };