$HoA{$rboc}{$telco}++; is fine, except it's a HoH, not a HoA. It's a shortcut for

$HoA{$rboc} = {} unless defined $HoA{$rboc};
my $anon_temp = $HoA{$rboc};
$anon_temp->{$telco}++
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$HoA{$rboc, $telco}++;
also works, but it means
$HoA{join($;, $rboc, $telco)}++;
which is probably not what you want.

Update: s/unless exists/unless defined/