First of all, the line:

my $line = $self->SUPER::getline() || return undef;
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has a problem; what if $self->SUPER::getline() returns "0"? Or ""? (which may happen if you don't subclass from IO::Handle directly, or if you are working with IO layers).

As for what to return if the line is illformatted, you can do several things:

• Return 'undef' after setting $! or some other global variable, and leave it to the caller to inspect it.
• die with an appropriate message; it's up to the caller to use eval {} and catch an error.
• Return an overloaded object that is undefined in boolean context, but which can be inspected by the caller. You might return an overloaded object if your superclass returns undefined as well.
• Always return an object, whether on failure or success. It's up to the caller to inspect that object, and retrieve either the string, or the failure reason.

Anyway, you need to return something special, which you document well, and it's up the caller to inspect your return value and handle appropriately.

I probably would use the first suggestion.

Abigail