You can read from STDERR. It's just as easy as <STDERR>.

Correction:

• You need to dupe STDERR as a readable file handle first, and this doesn't work in later versions of perl (5.8 and later won't allow you to dupe a write-only file-handle into a readable filehandle... although even by the 5.0 docs, it shouldn't be allowed (is my face red, or what))

  [me@host bin]$ perl -we 'open STDERRIN,"+<&STDERR" or die $!; my $line
  + = <STDERRIN>;  print STDERRIN "YAY!!! $line";'
  asdfasf
  YAY!!! asdfasf
  [me@host bin]$
  [download]

If you are not redirecting, then STDIN, STDOUT and STDERR are really all references to the same thing, your terminal. You can see this by running (at least on linux):

ls -l /proc/self/fd/
[download]

as well as variations on the theme, like:

ls -l /proc/self/fd/ < /dev/null
ls -l /proc/self/fd/ | cat
ls -l /proc/self/fd/ 2> /dev/null   #redrect STDERR
[download]

0 = STDIN, 1 = STDOUT, and 2 = STDERR.

If you want to be clever, then you can try to detect which of STDIN, STDOUT and STDERR are actually terminals, by using the -t operator. Also, you could just reach around the standard IO channels and open a direct shot to the controlling tty of the process like:

open TTY, ">/dev/tty" or die "Couldn't open /dev/tty $!";
[download]

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