I pretty much copied and pasted as well and came to the same conclusion as bjelli in her comment.
Just added a few things on real quick to make an inclusive script as well and was able to run without issue.

#!/usr/bin/perl -w

use strict;

use CGI;
my $q = new CGI;
my $subbut = $q->param("submitbut");

my @inventory = qw(books paper pencils erasers);

print $q->header;
print $q->start_html();

my %qty=();  my %inv_prices=();  my %inv_weights=();
if ($subbut eq 'Submit') {
  foreach (@inventory) {
    my $qty = $q->param("quantity$_");
    $qty{$_} = $qty;
  }
  print "<br><br><TABLE>";
  foreach (@inventory) {
    print "<TR>";
    print "<TD>$_</TD>";
    print "<TD>$qty{$_}</TD></TR>";
  }
  print "</TABLE>";
} else {
  print "<form><table>\n";
  foreach (@inventory) {
    print "<TR>";
    print "<TD>$_</TD>";
    print "<TD>$inv_prices{$_}</TD>";
    print "<TD>$inv_weights{$_}</TD>";
    print "<TD><input type=\"text\" size=\"4\" name=\"quantity$_\"></T
+D></TR>\n";
  }
  print "<TR>";
  print "<TD><input type=\"submit\" name=\"submitbut\" value=\"Submit\
+"></TD>\n";
  print "<TD><input type=\"reset\" value=\"Clear\"></TD></TR></table><
+/form>\n";
}

print $q->end_html;
[download]

nandeya