Without knowing more about SuffixTree, I can only come up with:

{
   local *STDOUT = *MY7;
   print_tree($tree);
}
[download]

Update: I see this is module is on CPAN (despite beingin the root namespace!) and that the print_tree function uses C (not Perl) printf calls. As such, this probably doesn't work. I don't know if any of the solutions here with work. If the three already submitted don't work, you'll have to redirect the output from the shell (perl script.pl > mississippi_out_7.txt, because the module doesn't allow you to specify an alternate output file.