You almost gave the answer yourself. The (?i:...) construct can be used to "cloister" (?) the modifiers. Try this:

%  perl -ne 'print +(/(?i:var)=value/)?"Ok\n":"Not Ok\n"'
var=value
Ok
Var=value
Ok
var=Value
Not Ok
VaR=value
Ok
VaR=valuE
Not Ok
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--ZZamboni