Since perl 5.13.2 s/// and tr/// support the /r flag, which returns a copy instead of the number of substitutions. The /r feature is called Non-destructive substitution

$ perl -MData::Dump -e " @f = 1..4; dd [ map { s/^/F/r; } @f ]; dd\@f;
+  "
["F1" .. "F4"]
[1 .. 4]
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