In the future (and in general cases where data structure isn't known), a simple

use Data::Dumper;
warn Dumper \%ARGS;
warn Dumper $ARGS{LineItem};
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would have show that it was an array ref, leading immediately to the @{$ARGS{LineItem}} solution.

Also, in mason you can do this:

<%args>
        $LineItem=>[]
</%args>
<%init>
  warn join ":", @$LineItem;
  ...
</%init>
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That takes care of the case where just one LineItem form element exists. Using that syntax, you'll still get an array ref. Using the above $ARGS{LineItem} syntax you'll get a scalar, which will make the @{$ARGS{LineItem}} solution error out.