Unfortunately,

1. There is no guarantee that the default interface actually connects to the internet (stupid, but not unknown)
2. Some OSes can have multiple default routes

A much easier method (and avoiding shelling out) is to create an IO::Socket::INET connection and then ask it which IP it binds to:

use IO::Socket::INET;
my $sock = IO::Socket::INET->new('www.google.com:80') 
                             or die "Can't bind : $@\n";
print 'IP Address with gateway to internet: ',$sock->sockhost(),"\n";
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Note that this does not identify which (physical or virtual) NIC has connectivity, but then, neither does the original code, so I am not sure that that was what was actually sought. IO::Interface offers functions to make that connection.