What you want in your snippet there is a grep() not map() as you're filtering out items not applying transforms to them e.g

shell> perl -le '%a = qw/1 1 2 2 3 3/; %b = qw/2 2 3 3/; \
                 print grep !$b{$_}, keys %a'
1
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Also note that the map() in your snippet is returning a list of three items - a 1 and two empty strings.
HTH

_________
broquaint