Syntactically, you need an expression here, not a statement, so you could use the ternary operator:

...
for
   qq(p4 sync -k "$options{v}"),
   exists $options{f} ? qq(p4 -x "$options{f}" delete) : (),
   exists $options{a} ? qq(p4 -x "$options{a}" add) : (),
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