Russ mentioned that the // operator is in Perl 6, but in fact it is available in Perl 5.10:

use 5.010;
$CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE") // 'sha1';
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This only puts 'sha1' in your config value if your $cfg->val returns undef or nothing.

Definitely a valuable addition to the language.