How to pass array from .launch file to .xacro file?

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The likely problem is that your arg values contain space characters (and note: this mechanism passes strings, not arrays):

command="$(find xacro)/xacro --inorder
                   '$(find <pkg>)/urdf/<file>.urdf.xacro'
                   pose_trans:=$(arg pose_trans)
                   pose_rot:=$(arg pose_rot)"

Your syntax expands to a command string like this:

/some/path/xacro --inorder /another/path/urdf/myfile.urdf.xacro pose_trans:=0 1 2 pose_rot:=3 4 5

Those digits with a space in front are the cause of the error message. One way to fix this in the launch file is to add single quotes around the entire arg expression, like this:

command="$(find xacro)/xacro --inorder
                   '$(find <pkg>)/urdf/<file>.urdf.xacro'
                   'pose_trans:=$(arg pose_trans)'
                   'pose_rot:=$(arg pose_rot)' "

The single-quotes will be stripped off before xacro sees them, so you shouldn't need to change anything in your xacro file.

I have discovered a working solution. It's a little bit of a hack.

In your launch file do:

<origin xyz="$(arg pose_trans)" rpy="'$(arg pose_rot)'" />

In your .xacro do:

<xacro:arg name="pose_rot" default=""/>
<xacro:property name="pose_rot" value="${$(arg pose_rot)}"/>

Why does this work?

xacro: error: expected exactly one input file as argument

This error is caused by the args being passed to the xacro command in plain text without quotes. By adding an apostrophe style quote '' in my version, you can pass your array string as usual. This is because roslaunch is providing your string arg to the command without quotes by default (e.g. xacro xyz.urdf.xacro pose_rot:=[1, 2, 3] instead of `xacro xyz.urdf.xacro pose_rot:='[1, 2, 3]'). An alternate solution here is to just leave out the spaces in your array that you are passing.

The second piece we make use of in the xacro is the yaml parser contained in the xacro documentation. Specifically, when we use ${} around a string that is formatted as an array, the xacro parses it like it does a yaml array.