I'm not answering your question as I never play with SOAP. I just wanted to let you know that you don't need that multiple assignments and superflous double quotes on literal strings. Instead, you can use <<here-doc syntax:

my $post = <<EOF;
<request>
<Number>$number</Number>
<InternalData></InternalData>
</request>
EOF
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But, where the heck that $number variable comes from?