I don't see why you should ever use a literal undef when you really should be using an empty string,

Sorry to 'go all CS on you' :), but of all people I thought you would see the utility in distinguishing between bottom (or null; or undef) and a variable that current has length zero.

In general, Perl is pretty good about DWIMing when we attempt to perform (legitimate) actions on variables that are currently set to undef.

my $i; ++$i, print $i;
1
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Inventing na empty string to say that I want the target substring to be replaced by nothing just seems...clumsy.

But it's no big deal. (Really, it isn't). Just a random thought that

my $record = substr $data, 0, $CDRLEN, undef;
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might more clearly convey the semantics of the underlaying operation than

my $record = substr $data, 0, $CDRLEN, '';
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And that's it. Personally, I think it does, but is currently hampered in use by the warning. And it struck me that warning could be suppressed where it is obviously a rational choice.