What is the correlation between 'b' and $b? Is one used to derive the other? If so, then map may help. 
$a = {
    map {
        $val = some_derivation($_)
            ? ( $_ => $val ) 
            : ()
    } qw(a b c)
};

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Ivan Heffner
Sr. Software Engineer, DAS Lead
WhitePages.com, Inc.

In reply to Re: Shortcut operator for $a->{'b'}=$b if $b; by Codon
in thread Shortcut operator for $a->{'b'}=$b if $b; by rokadave

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