Surely, this is a question susceptible to just a little bit of experimentation:

>perl -wMstrict -le
"my $which;
 my $s = 'foo' . $which ? 'bar' : 'baz' . 'boff';
 ;;
 $s = 'foo' . ($which ? 'bar' : 'BAZ') . 'boff';
 print qq{'$s'};
 ;;
 $which = 1;
 $s = 'fee' . ($which ? 'FIE' : 'foe') . 'fum';
 print qq{'$s'};
"
Use of uninitialized value $which in concatenation (.) or string ...
'fooBAZboff'
'feeFIEfum'
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