use strict;
use warnings;

use IO::Prompt;

while( prompt "next: " ) {
  print "You said '$_'\n";
}
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$ cat > test.txt
Hello
World
^D
$ perl ./prompt.pl < test.txt
You said: Hello
You said: World
$ cat test.txt | perl ./prompt.pl
You said: Hello
You said: World
[download]

Update: I missed the point. If you want to perform differently if the input is given in a pipe or not, you can use IO::Interactive:

use strict;
use warnings;

use IO::Interactive qw(is_interactive);

if (is_interactive){
  print "interactive\n";
} else {
  print "piped\n";
}
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If you save this script as "interactive.pl":

$ perl ./interactive.pl
intereactive
$ cat test.txt | perl ./interactive.pl
piped
$ perl ./interactive.pl < test.txt
piped
[download]

citromatik