I was going to try to explain the reason for this feature, but then I thought, I bet it's in the documentation for the error message.

`perldoc perldiag`

       In string, @%s now must be written as \@%s
           (F) It used to be that Perl would try to guess whether
           you wanted an array interpolated or a literal @.  It
           did this when the string was first used at runtime.
           Now strings are parsed at compile time, and ambiguous
           instances of @ must be disambiguated, either by
           prepending a backslash to indicate a literal, or by
           declaring (or using) the array within the program
           before the string (lexically).  (Someday it will
           simply assume that an unbackslashed @ interpolates an
           array.)
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So, the reason for this error message is to flag a change in the implementation that will be incompatible with earlier versions of Perl.