s/_/\_/g

s/_/\_/g does nothing. You probably want  s/_/\\_/g

>perl -wMstrict -le
"$_ = 'a_b__c___';
 print qq{before: '$_'};
 ;;
 s/_/\_/g;
 print qq{after:  '$_'};
"
before: 'a_b__c___'
after:  'a_b__c___'
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But please allow me to add my voice to the chorus imploring you to Just Use Placeholders!.