tie my %h, Tie::Hash::DefaultVal => "default-val";
%h = (
    foo => "foo-val",
    bar => "bar-val"
);

print "\$h{$_} = $h{$_}\n" for qw[ foo bar nonexistant ];

__OUTPUT__
$h{foo} = foo-val
$h{bar} = bar-val
$h{nonexistant} = default-val