The first arg of printf should be a formatting string. 

printf("%s", "$line\n");  # ok
printf("%s\n", $line);    # ok
printf("$line\n");        # ERROR

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You want print. 
print("$line\n");

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In reply to Re: Usage of File::Tail::Multi generates conversion errors by ikegami
in thread Usage of File::Tail::Multi generates conversion errors by WaywardBuddha

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