Without you specifying in more detail how expr1,expr2,expr3 are to be plugged into your CASE...END statements, I can only offer this

while(<>){
    if( /IF\(expr1,expr2,expr3\)/ ) { 
        if( rand() > 0.1 ) {
            s/IF\(expr1,expr2,expr3\)/CASE\nWHEN condition THEN result
+\n[ WHEN condition THEN result ]...\n[ ELSE result ]\nEND\n/;
        } else {
            s/IF\(expr1,expr2,expr3\)/CASE expression\nWHEN value THEN
+ result\n[ WHEN value THEN result ]...\n[ ELSE result ]\nEND\n/;
        }
    }
}
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