Thanks to all of you
My purpose was to browse incomming records one at a time, and let the user decide if he wanted to see one more.
I combined several of the ideas suggested here:

• using <> for reading incomming records, so I don't have to care if data comes from pipe or redirection or files supplied on command line,
• -t STDIN for determining if STDIN is a terminal,
• open /dev/tty to get the user input (which was my original problem)
• and Term::Readkey to avoid the need of hitting an extra RETURN.

Finally the script turned out like this (only skeleton showned):

#!/usr/bin/perl
use strict;
use warnings;
use Term::ReadKey;

# Is data comming?
usage() unless ( ( ! -t STDIN ) || ((defined($ARGV[0])) && ( -f $ARGV[
+0] )) );
open(TTY,"+</dev/tty") or die "Couldn't open tty: $!\n";

# input record separator is \x1d
$/ = "\x1D";

while ( my $rec = <>) {
  next unless defined $rec;
  show_formatted_record();

  print TTY "\n\nOne more? y/n ";

  ReadMode('cbreak', *TTY);
  my $answer = ReadKey(0, *TTY);
  ReadMode('normal', *TTY);

  last if $answer !~ /^[yY]$/;
}
print "\n";

#---- functions: 
sub usage { print "usage: ...\n"; exit 1; }

sub show_formatted_record { print "..."; }
[download]

Thanks again,
L