I'm not squemish about compound statements, and I like to avoid unnecessary code, but this makes me wince:

return $pkg->{'DIST'} = 
    ( $rh = $pkg->{'RPM_HDR'} and $dist = $rh->tag('DISTRIBUTION') )
    ?    $dist 
    :    $undef;
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It should work, but since you need two temporary vars which have to be declared (you do use strict don't you;), then this seems much cleaner:

$pkg->{'DIST'} = undef;
my $rh = $pkg->{'RPM_HDR'} or return;
my $dist = $rh->tag('DISTRIBUTION') or return;
return  $pkg->{'DIST'} = $dist;
[download]

Probably minutely more efficient to boot.