You might want to use $eth1 and $eth2 instead of re-using the variable $eth. That said, have you investigated the created object to confirm it is what you think it is?

print $eth1->print, "\n";
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It sounds like your frames are being sent to the default MAC. From Net::Packet::Env, it looks like you might want to do something more like:

my $env1 = Net::Packet::Env->new(dev => 'eth1');
.
.
.
my $frame = Net::Packet::Frame->new(env=>$env1,l3=>$ip6,l4=>$udp,l7=>$
+data);
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