If zero or the empty string aren't valid values then you could use:

$CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE") || 'sha1';
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Otherwise, you'd need:

$CONFIG{HASHTYPE} =
  defined $cfg->val("MISC","HASHTYPE")
    ? $cfg->val("MISC","HASHTYPE")
    : 'sha1';
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which isn't much different to what you've got.