Generally publishers release new editions because they need to get some more copies of a book out there, but the world has changed so they want to give the author a chance to update things before going to the printer again. When a publisher makes big changes, they usually highlight it prominently in the marketing material to get people to buy the new edition even if they have the old one.
In the case of an intro book, the question is "If someone was learning Perl today, what would you want to tell them?" The answer changes slightly as time goes on because Perl changes, but it's not a major change.
As others have mentioned, you have already been introduced to Perl, so you probably don't need the newer version.
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