#!/usr/bin/perl -w
use strict;
use warnings;
use String::CRC32;         ### for instance 

my $XmlData = <<XML_DATA;
<Transaction>
    <MessageCode>100</MessageCode>
    <ToAccountNo>12989898900</ToAccountNo>
</Transaction>
<Transaction>
    <MessageCode>200</MessageCode>
    <ToAccountNo>24536485582</ToAccountNo>
</Transaction>
XML_DATA

open my $XmlFile, '<', \$XmlData or die "Can't open XML file: $!";

$/ = "</Transaction>";    ### one trans at-a-time
while ( <$XmlFile> ) {
    my  $crc = crc32($_);
    ### & do whatever needs TBD with the $crc ...
}