sub new {
    my $class = shift;
    my %vars = @_;

    $vars{'error'}      = '';

    $vars{'reply'}      =  {
        'status'        => 'noaction',
        'sent_to'       => [ ],
        'sent_to_all'   => 'false',
    };
    
    if (!exists{'payload'}) {
        # Obtaining  payload from STDIN only 
        # exists for backward  compatability
        # This option is deprecated and will
        # be  removed  in  a  future version
        if (exists $ENV{'GATEWAY_INTERFACE'}) {
            read(STDIN, $vars{'payload'}, $ENV{'CONTENT_LENGTH'});
            $vars{'webhook'} = decode_json($vars{'payload'}) if $vars{
+'payload'};
            $vars{'error'}   = 'No payload data' unless $vars{'webhook
+'};
        } else {
            $vars{'error'}   = 'Looks like this is not a web request!'
+;
        }
    }
    
    return bless \%vars, $class;
}
[download]

It seems clearer and easier to test if this module were to simply accept a payload property

Is that a better constructor?
The production version of the module has been released so I don't want to break that. So the new method now accepts a payload property. If that is missing and it is running on a webserver with the right environment, it falls back to the previous behaviour.

A couple of versions down the line I will remove the fallback code and set an error condition if payload is not a valid JSON scalar.