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package Tie::Hash::DefaultVal;
require Tie::Hash;
...
    my ($self, $key) = @_;
    exists $self->[0]{$key} ? $self->[0]{$key} : $self->[1];
}

tie my %h, Tie::Hash::DefaultVal => "default-val";
%h = (
...
$h{foo} = foo-val
$h{bar} = bar-val
$h{nonexistant} = default-val