in reply to Q: NaN again

Example of "NaN script": 
#!/usr/bin/perl
use CGI;

my $q = new CGI;

my $summ = 100;
my $peritem = 1;

print "Content-type: text/html\n\n";

unless ($q->param('buyit')) {
    print "<HTML>You have $summ coins.<BR>All items cost $peritem coin
+.<FORM>Enter number of items to buy<INPUT type=text name=items><input
+ type=SUBMIT name=buyit value=go></FORM>";
} else {
    my $num = int($q->param('items'));
    my $cost = sprintf("%.2f", $num * $peritem);
    print "<HTML>You try to buy $num items, for $cost coins.<BR>";
    if ($cost > $summ) {
        print "You have not enough money<BR>";
    } elsif ($cost < 0) {
        print "Invalid number<BR>";
    } else {
        $summ-=$cost;
        print "You buy $num items for $cost coins. You have $summ coin
+s left<BR>";
    }

}

[download]
Results: 
** 23 coins:
You try to buy 23 items, for 23.00 coins.
You buy 23 items for 23.00 coins. You have 77 coins left

** 102 coins:
You try to buy 102 items, for 102.00 coins.
You have not enough money

** -3 coins:
You try to buy -3 items, for -3.00 coins.
Invalid number

** nan coins:
You try to buy nan items, for nan coins.
You buy nan items for nan coins. You have nan coins left

[download]

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