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

sub sqroot {
   my $maxdiff = 1e-18;
   my ($value, $num) = (1, shift @_);
   my $i = '';   # Added
   $num *= -1 and $i = 'i' if ($num < 0);  # Added
   my $oldvalue = 0;
   while (abs($oldvalue - $value) > $maxdiff) {
      $oldvalue = $value;
      $value = $num / $value;
      $value = ($value + $oldvalue) / 2;
   }
 # return $value;  #  (old)
   return $value . $i;
}

my $num;
do {
   print "Please enter a number: "; # changed from "positive number"
   chomp ($num = <STDIN>);
# } until ($num =~ /^\d*[.\d]\d*$/);  #  (old regex)
} until ($num =~ /^-?\d+\.?\d*$/);

my $sqrt = sqroot $num;
print "Square root of $num: $sqrt\n";