It's easy enough to get the current size of the file:

  my $current_file_size = -s $file_name;
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So you just need to have that value, and check it against the number that the user provides as the offset to go to:

  if ( $start_pc >= $current_file_size ) {
     warn sprintf( "Your requested offset (%d) is past EOF (%d)\n",
                   $start_pc, $current_file_size );

     # and do something else, like prompt for new input
  }
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Of course, if the user is asking for fixed-length-record $n, you'd want to say that the requested $n is greater than the number of records in the file (file size divided by record size, which, in an ideal world, would always be an integer value).