Essentially this is a trivial use of XML. The data could just as easily have been sent as Field: Value. You could quite easily just interpolate values into a post string to get the required format for the post data and then use LWP::UserAgent to do the POST and accept the REPLY. To parse the sort of trivial reply you have to deal with you could use XML::Simple. If you are certain that the XML will always be well formed you could use a simple RE (but this is not recommended)....

while(<DATA>){
    next unless m!^\s*<([^>]+)>([^<]+)</(\1)>!;
    $h{$1}=$2;
}
use Data::Dumper;
print Dumper \%h;

__DATA__
<Reply>
<TransactionReply>
<ResponseCode>0</ResponseCode>
<ResultCode>00</ResultCode>
<ResultMessage>APPROVED</ResultMessage> 
<TransactionID>100000182165</TransactionID>
<AVSResponseCode />
<CVVResponseCode />
<ApprovalNumber>123456</ApprovalNumber>
<AuthorizationNumber>123456</AuthorizationNumber>
<TransactionDate>021112</TransactionDate>
<TransactionTime>110544</TransactionTime>
<ReferenceNumber>1234</ReferenceNumber>
<AccountNumber>XXXXXXXXXXXX1111</AccountNumber>
<TransactionAmount>1.00</TransactionAmount> 
</TransactionReply>
</Reply>
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