What's considered a character special to the shell is *not* the same as all characters not matching /[A-Za-z_0-9]/. This is immediately obvious from the example of the OP. Both examples contain a space, a space is not matched by /[A-Za-z_0-9]/, yet in one case the shell is called, in another it isn't.

Here's another example:

    $ perl -wle 'exec ", foo" or die'
    Can't exec ",": No such file or directory at -e line 1.
    Died at -e line 1.
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No shell called, even with a comma present.

Abigail