This is a common thing. Various Linux tools do this all the time, so I'm not sure why the "standard" answer wasn't posted within minutes. /dev/tty is an alias for the controlling terminal: 
while (<>) {
  $stream .= $_;
}

open TTY, "</dev/tty";
print "Do you want to process the stream?";
my $ans = <TTY>;

[download]
Alternatively, you can actually read from STDERR, just as you might print status or error messges to STDERR. If your reading inout from stdin, you might want to be able to write output to stdout too, rather than having your prompts go to stdout. You can print your prompts to STDERR or open the tty read/write "+</dev/tty".
~~~~~~~~~~~~~~~
Ray Morris
support@webmastersguide.com

In reply to Re: Multiple STDIN sources by raymor
in thread Multiple STDIN sources by perlfan

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.