I tried out your code after eliminating the XML and mysql-dbi - it seemd to work.

changed the lwp code to hard-code:

my $post_data1 = "first_name";
my $post_data2 = "Last name";
my $post_data3 = "10/10/1999";
my $post_data4 = "someone\@somwehere.com";
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When the LWP is run, I get:

Received reply: first_name was received
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Perhaps the content retrieved from XML does not match your code's expectations.