You also need to be careful about your use of the semi-colon:

$mystring .= << "START";  # semi-colon needed here

<XML version="" attribute2="" ...blah

START
# Don't put a semi-colon on that line (or a comment or anything else).
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The end of the here-doc must be a line that contains exactly and only the string that you gave at the start, excluding the quotes (if any) and the semi-colon.