<r>
  <ch>
    <PortDetails>
      <PortType>A</PortType>
      <PortCode>USBAL</PortCode>
      <CutOffDate>2016-09-28</CutOffDate>
      <CutOffTime>12:00</CutOffTime>
    </PortDetails>
  </ch>
  <ch>
    <PortDetails>
      <PortType>L</PortType>
      <PortCode>USMIA</PortCode>
      <ETD>2016-10-13</ETD>
    </PortDetails>
  </ch>
</r>
[download]

and used the following xsh script to create a new file:

my $in  := open { $ARGV[0] } ;
my $out := create RoutingDetails ;
my $rd  =  $out/RoutingDetails ;

my $pda = $in//PortDetails[PortType='A'] ;
if $pda {
    set $rd/StageQualifier 1 ;
    set $rd/TransportMode  1 ;
    set $rd/TransportName  'NV PRECARRIAGE' ;

    insert element Origin into $rd ;
    cp  $pda/PortCode/text() into $rd/Origin ;

    insert element ETD into $rd ;
    cp $pda/CutOffDate/text() into $rd/ETD ;
}

my $pdl = $in//PortDetails[PortType='L'] ;
if $pdl {
    insert element Destination into $rd ;
    cp  $pdl/PortCode/text() into $rd/Destination ;

    insert element ETA into $rd ;
    cp $pdl/ETD/text() into $rd/ETA ;
}

save :f 'output.xml' $out ;
[download]

The output was different to what you stated, because it correctly used the values present in the input.

<?xml version="1.0" encoding="utf-8"?>
<RoutingDetails>
  <StageQualifier>1</StageQualifier>
  <TransportMode>1</TransportMode>
  <TransportName>NV PRECARRIAGE</TransportName>
  <Origin>USBAL</Origin>
  <ETD>2016-09-28</ETD>
  <Destination>USMIA</Destination>
  <ETA>2016-10-13</ETA>
</RoutingDetails>
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]