First, your $errorstate variable is named the opposite of what it should be. You set it to 1 when the input is formatted properly! Then, the bottom half of your code can be idiomized using some built-in variables in Perl. And are you sure you want to REMOVE the '#' from lines starting with '#port'? You also do some unnecessary work of checking to see if a regex matches before doing a substitution using that regex.

It turns out you can get rid of $errorstate altogether, and just loop until $sshport has a value.

#!/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 extensio
+n" 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
  }
}
[download]

See perlvar for information about $^I.