in reply to Re^2: Appending strings
in thread Appending strings

It may be the /r modifier to s///, which was added in perl version 5.14: Regular Expressions. What perl version are you on? 
perl -v

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Re^4: Appending strings
by doofus (Acolyte) on Sep 17, 2015 at 00:48 UTC
    This is perl, v5.8.8 built for x86_64-linux-thread-multi
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