Note that in your original post you had parentheses around the subroutine, these is also a slight gotcha in this example if you do:

 perl -e'print (localtime())[5]'
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as the parser takes the outer parentheses to be the argument list of the print which obviously can't be subscripted meaningfully, in this case you will want to use the no-op unary plus to disambiguate the context of the subscript:

 perl -e'print +(localtime())[5]'
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/J\