Now the following works, and prints "read in 'xxx'". Given the file io.txt:

  onetwothreefour
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and the following code:

  #! /usr/bin/perl -w
  use strict;

  if (open FILE, '+<io.txt')
  {
    seek FILE, 3, 0;
    print FILE 'xxx';

    seek FILE, 3, 0;
    my $input = '';
    sysread FILE, $input, 3;
    print "read in '$input'\n";

    close FILE;
  }
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But you really ought to have provided the misbehaving code.