That's because of the || ''. If the substitution returns false, i.e. there's no smart quote to substitute, you replace this return value that goes nowhere ("void context") by an empty string. You probably meant to add it into the assignment instead? 
map { (my $dx = $data->{ $_ } || '') =~ s/’/'/g; $dx } @allparms

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But that would replace 0's with empty strings. Use // to replace only undefs.

Update: 

map { (my $dx = $data->{ $_ } // '') =~ s/’/'/g; $dx } @allparms

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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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In reply to Re^4: Replacing single quotes to two single quotes inside map by choroba
in thread Replacing single quotes to two single quotes inside map by Anonymous Monk

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