It is something new with 5.6: 
% perl -v

This is perl, v5.6.0 built for i686-linux

   [snip]

% perldoc perldelta

   [snip]

       exists() is supported on subroutine names
 
       The exists() builtin now works on subroutine names.  A
       subroutine is considered to exist if it has been declared
       (even if implicitly).  See the exists entry in the
       perlfunc manpage for examples.

   [snip]

[download]
The perldelta documentation is the place to find out about significant changes in the latest version of perl. perldeltas from previous versions are also included in a new install, but renamed to include the version, e.g. perl5004delta.

In reply to Re: thanks guys by chipmunk
in thread parsing parameters in a new() object call by YaRness

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