Be sure to always check return values.
I hope you find this rewrite helpful -- I think it's roughly equivalent to what you're trying to do.
Notice how easily the CGI.pm can handle the reading and writing for you :-)

#!/usr/bin/perl -w

# -w to turn on warnings

# XXX These should be fully qualified paths
our $DATA_FILE = 'character.data';
our $FORM      = 'save.htm';

use strict; # you should almost always use this
use IO::File;
use CGI qw/ -debug /;

# parse the user's submission
my $q = CGI->new;

# fetch previously stored data
my $fh = IO::File->new($DATA_FILE) or die "unable to read '$DATA_FILE'
+: $!";
my $disk = CGI->new($fh);

# overwrite values
foreach my $key ($q->param) {
  $disk->param($key, $q->param($key));
}

# XXX What happens when two users run this script simultaneously?
# Answer: data corruption. We need locking.
$fh = IO::File->new("> $DATA_FILE") or die "unable to write '$DATA_FIL
+E': $!";
$disk->save($fh);

# XXX Better to move the common code into a module.
# How do we know if system() worked?  See $? in perlvar
# This should be fully qualified too.
#system 'menu.cgi'             ;

my $out = join "\n", map { "$_=" . $disk->param($_) } $disk->param;

$fh = IO::File->new($FORM) or die "unable to read '$FORM': $!";
print $q->header, <$fh>, $out;
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