Splunkで使える比較演算子は6つなので、力技ですがcase文に全ていれてしまってはどうでしょうか。
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| eval i = 6
| eval flag=case(affected="=",if(value=i,"true","false"),affected="!=",if(value!=i,"true","false"),(affected="=<" OR affected="<="),if(value<=i,"true","false"),(affected="=>" OR affected=">="),if(value>=i,"true","false"),affected="<",if(value<i,"true","false"),affected=">",if(value>i,"true","false"))
| where flag="true"
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