If we can assume that file is a variable containing the old name of the file, and timestamp is a variable containing the, well... timestamp... you may find that one would access the value of a variable named file by prepending a dollar sign ($), not an ampersand (&). An ampersand generally denotes a subroutine, while a dollar sign denotes a scalar variable.

You might also want to post a more complete version of your code for further dissection.

$/  =  q#(\w)#  ;  sub  sig { print scalar reverse  join  ' ',  @_  } 
+ sig
map { s$\$/\$/$\$2\$1$g && $_ } split( ' ', ",erckha rlPe erthnoa stJu
+" );
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