read is not guaranteed to read the number of bytes you specify, especially when dealing with handles which aren't tied to files. I also added error handling and made the buffer size configurable. 

use constant BLK_SIZE => 16*1024;

sub pseek {
   my( $p, $to_read, $blk_size ) = @_;
   $blk_size ||= BLK_SIZE;
   while ( $to_read ) {
      $blk_size = $to_read if $to_read < $blk_size;
      my $read = read( $p, my $discard, $blk_size );
      return $read if !$read;
      $to_read -= $read;
   }
   return 1;
}

[download]

Update: Or maybe not. My testing shows that read does wait, but its documentation uses the same wording as sysread which does not. As such, I wouldn't count on the observed bahviour. 

$ perl -e'$|=1; print "a"; sleep(10); print "b"' | perl -le'read(STDIN
+, $buf, 10); print $buf'
ab
$ perl -e'$|=1; print "a"; sleep(10); print "b"' | perl -le'sysread(ST
+DIN, $buf, 10); print $buf'
a

[download]

Same results on linux and Windows.

In reply to Re^2: seek() functionality on pipes by ikegami
in thread seek() functionality on pipes by HKS

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