"perldoc perlvar" command will say about such funny variables in detail, but in short why you did not noticed any distinctions when changing array bound base -- you did not relied on that in your script!

Namely, you filled entire array with values, and after that you may be output that array out entirely, without specifying any bound at all.

You'll see a diffenrence when you'll do some implicit indexing, such as $in_data[0] or somesuch.

And remember once again, changing $[ is deprecated.

update:: fixed a typo ($[ instead of $])

Have a nice day,
Jim