Firstly, I don't like the idea of overloading join as reduce. They're two quite different functions. But others have already told this, so I'll also show something else.

As I see, the builtin functions of perl are almost frozen and almost no change has been done on them till 5.004.

• There's been a general rework of open (three-arg open and vvarious other extensions).

Apart from that, the only ones I know of are implications of how builtin data-types have changed:

• delete and exists now work on arrays,
• hash functions respect restricted hashes,
• file functions work on lexical handles and autovivify them,
• string functions (including pack and unpack) work on both character and byte strings depending on the utf8 flag,
• binmode and open now accepts perlio layers,
• my and sub accepts attributes.

To back up my statement, I've compared the perlfunc of perl 5.004 and 5.8.8, and here are the other significant changes of semantics I've found.

• The argument of close is optional and defaults to the selected handle.
• The argument of cos, eval is optional and defaults to $_ (or this got documented because sin was already like this, I don't know).
• The argument of dump is optional and I don't care what it defaults to.
• Exec now accepts an indirect argument to tell which program to run, if not $ARGV[0].
• The argument of localtime is optional and defaults to time (so that's why I see localtime(time()) so often).
• Lock is made a new builtin function.
• The second argument of mkdir is optional and defaults to 0777.
• No now accepts a version (with or without a module; use did this already).
• Our is a new builtin function.
• Pack and unpack accepts some new templates: parenthesis, q Q j J F D, bracketed repeat counts, exclamation mark, x!.
• The argument of package can be omitted but this feature is depreceated.
• Prototype works for builtin functions.
• Qr precompiled regular expression quoting is introduced.
• Sort accepts a ($$) subroutine instead of implicitly passing values in $a and $b.
• You can now call splice with only one arguments, when it would remove all elements (no-one knows this, so this is a nice obfu-feature).
• Sprintf supports a new %b conversio to output an integer in binary, the v flag, the ll flag.
• Substr accepts a fourth argument as a replacement.
• Sysread accepts a fourth argument to specify an offset within the string (this is useful).
• You can omit the length from syswrite, which now deafults to the length of the scalar.

Of course, I could have easily skimmed over some other difference.

Update: listified changes. This way it seems much more.

Update: see also Builtin functions defaulting to $_