Well ... the way you've coded things, TT::s1() calls TT::s2(). If you don't want TT::s2() to be called then you need to either:
a) not call TT::s1(), or;
b) rewrite TT::s1() in such a way that it accommodates the possible existence of main::s2() - for example (in TT.pm):

sub s1 {
  print "s1";
  if(defined(&main::s2)) {main::s2()}
  else {s2()}
}
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See perldoc -f defined

Cheers,
Rob