Well, I know that the tr operator expects a list of characters, but some of them have special meaning such as the minus sign. Where perlop.html talks about the tr operator, it says,

"A character range may be specified with a hyphen, so tr/A-J/0-9/ does
+ the same replacement as tr/ACEGIBDFHJ/0246813579/. For sed devotees,
+ y is provided as a synonym for tr. If the SEARCHLIST is delimited by
+ bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, w
+hich may or may not be bracketing quotes, e.g., tr[[A-Z]][[a-z]] or t
+r(+\-*/)/ABCD/."
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I don't understand what this means. I just know that certain characters have special meanings following the tr operator, so to be on the safe side, I just put a backslash before each.