my %actions = (
    'sender a' => { 
        'some text' => { 
            'some other text' => 'type 1',
            'some other other text' => 'type 2',
        },
        'some other subject' => {
           ...
        },
    },
    'sender b' => {
        ...
    },
);

sub action {
    my( $sender, $subject, $body ) = @_;
    my $rv = $action{$sender} // return 'unknown sender';
    $rv = $rv->{$subject} // return 'unknown subject';
    $rv = $rv->{$body} // return 'unknown body';
    return $rv;
}