#!/usr/bin/perl

use strict;
use warnings;

my $sshport = 0;

until ($sshport) {
  print "Please enter SSH port number> ";
  chomp($sshport = <STDIN>);
  $sshport = 0 unless $sshport =~ /^[1-9]\d{1,9}$/;
}

{
  local $^I = ".old";             # the "in-place edit backup extension" variable
  local @ARGV = ("sshd_config");  # the files to edit in-place

  while (<>) {
    s/^(#?)port .*/$1Port $sshport/i;
    # this KEEPS the '#' character,
    # which I think is the correct behavior
  }
}