There's a module on CPAN designed exactly for this: DBIx::XML_RDB. You give it a SQL statement, and it executes the statement and dumps the returned rows and columns out in XML:

use DBIx::XML_RDB;

my $xmlout = DBIx::XML_RDB->new($datasource,
    "ODBC", $userid, $password, $dbname)
    or die "Failed to make new xmlout";

# tell DBIx::XML_RDB which data it should retrieve
# (by using a SELECT statement)
$xmlout->DoSql("select id, name from foo");

# write out the data in XML format
print $xmlout->GetData;
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The module installs two utility scripts: sql2xml dumps SQL rows into XML, and xml2sql does the opposite.