The problem seems to be that you are reading the next Instrmt element in a while loop and as a result you skip the parent MktDataFull elements.

If you change the while() to an if() it should fix the main issue.

    ...
    if ( $reader->nextElement( 'Instrmt' ) ) {
        my $Sym = $reader->getAttribute( 'Sym' );
        ...
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--
John.