You have to consume the argument in @ARGV before the implicit loop starts. What's more, you must make sure the argument is consumed only once. Hence, most elegant solution: do it in a BEGIN block. 
echo stuff | perl -ne 'BEGIN{ $x=shift @ARGV; } print "x=$x\n"' myarg

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Is this like what you had in mind?

In reply to Re: Differentiating STDIN from arguments when using -n ? by bart
in thread Differentiating STDIN from arguments when using -n ? by rduke15

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