As you can see below, the = binds tighter than the and, so your second example is parsing as:

(my($yes2)=defined $bad) and -f $bad;
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Deparse:

[~]$ perl -MO=Deparse -e 'my($yes2) = defined $bad and -f $bad;'
-f $bad if my($yes2) = defined $bad;
-e syntax OK
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