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
}
}
See
perlvar for information about $^I.