The * sigil refers to Typeglobs and Filehandles. In this case, the author wanted to pass the newly-opened XML filehandle into a subroutine, and old fashioned Perl didn't have a particularly clean way to pass one. Nowadays, you'd probably write that as:

 open (my $XML, "<", $xml_file) or die "File open failed ($xml_file): 
+$!;
get_data($XML, \%hash);
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The indirect filehandle makes passing and stashing much cleaner (IMHO). See also open and Indirect Filehandles in perlopentut.