(from commit log of perl5-current/Makefile):
* Interpreter cloning, threads, and concurrency
* Lexically scoped warning categories
* Unicode and UTF-8 support
* Support for interpolating named characters
* "our" declarations
* Support for strings represented as a vector of ordinals
* Improved Perl version numbering system
* New syntax for declaring subroutine attributes
* File and directory handles can be autovivified
* open() with more than two arguments
* 64-bit support
* Large file support
* Long doubles
* "more bits"
* Enhanced support for sort() subroutines
* C<sort $coderef @foo> allowed
* File globbing implemented internally
* POSIX character class syntax [: :] supported
* Improved C<qw//> operator
* pack() format 'Z' supported
* pack() format modifier '!' supported
* pack() and unpack() support counted strings
* Comments in pack() templates
* Weak references
* Binary numbers supported
* Lvalue subroutines
* Some arrows may be omitted in calls through references
* Boolean assignment operators are legal lvalues
* exists() is supported on subroutine names
* exists() and delete() are supported on array elements
* Pseudo-hashes work better
* Automatic flushing of output buffers
* Better diagnostics on meaningless filehandle operations
* Where possible, buffered data discarded from duped input filehandle
* eof() has the same old magic as <>
* binmode() can be used to set :crlf and :raw modes
* C<-T> filetest recognizes UTF-8 encoded files as "text"
* system(), backticks and pipe open now reflect exec() failure
* Improved diagnostics
* Diagnostics follow STDERR
* syswrite() ease-of-use
* Better syntax checks on parenthesized unary operators
* Bit operators support full native integer width
* Improved security features
* C<require> and C<do> may be overridden
* $^X variables may now have names longer than one character
* New variable $^C reflects C<-c> switch
* New variable $^V contains Perl version as a string
* Optional Y2K warnings
* Modules
* Pragmata
* dprofpp
* find2perl
* h2xs
* perlcc
* perldoc
* The Perl Debugger
* Simple sort() using { $a <=> $b } and the like are optimized
* Optimized assignments to lexical variables
* Faster subroutine calls
* -Dusethreads means something different
* New Configure flags
* Threadedness and 64-bitness now more daring
* Long Doubles
...
See 'perldoc perldelta' for a full list.
