To tie directly STDIN you can hold the original STDIN in other GLOB, soo this will work:

  *ORIGINAL_STDIN = \*STDIN ;
    
  tie *STDIN, 'Tie::Input::Insertable', *ORIGINAL_STDIN;
  
  print "> " ;
  while (<STDIN>) {
    print STDIN "Hello, world\n" if /foo/;
    print "\n# $_\n" ;
    print "> " ;
  }
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And by the way, nice work in the TIEHANDLE. ;-P