If you're worried about polluting other's namespaces, why don't you just override the function locally? I started with japhy's modification and produced this:

BEGIN {
   use subs 'length';
   sub length(;$) {
       my $arg = @_ ? $_[0] : $_;
       defined($arg) ? CORE::length($arg) : undef;
   }
}
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You could even modularize this pretty easily:

package Func::Smarter;
require Exporter;
use vars qw'@ISA @EXPORT @EXPORT_OK';
use subs qw 'length'

@ISA = 'Exporter';
@EXPORT_OK = qw'length';

sub length (;$) {
    my $arg = @_ ? $_[0] : $_;
    defined($arg) ? CORE::length($arg) : undef;
}

1;
[download]

Then you could just use Func::Smarter 'length';, yes?