From perldoc perlopentut:

Indirect Filehandles

"open"'s first argument can be a reference to a
filehandle.  As of perl 5.6.0, if the argument is
uninitialized, Perl will automatically create a
filehandle and put a reference to it in the first
argument, like so:

  open( my $in, $infile )
    or die "Couldn't read $infile: $!";
  while ( <$in> ) {
    # do something with $_
  }
  close $in;

It doesn't get much more dynamic than that.