$seed and $rand are indexes into @range, but @range changes so you need to get the value at that index.

use strict;
use warnings;

my $low = 1;
my $high = 10;
my @range=($low .. $high);
my $rand=$range[rand(@range)];     # <---
my $seed=$low-1;                   # <--- Works for all values of $low
+ and $high.
my $count=1;
my $lastguess;
while ( $seed != $rand )
{
    #undef @range if ( $ count > 1);  # <--- Useless.
    @range=($low .. $high);
    $lastguess = $seed;
    $seed = $range[rand(@range)];     # <---
    if ( $seed < $rand )
    {
        print "You're too low - $seed (last guess = $lastguess)\n";
        $low=$seed+1;
        $count++;
    }
    elsif ( $seed > $rand )
    {
        print "you're too high - $seed (last guess = $lastguess)\n";
        $high=$seed-1;
        $count++;
    }
}

print "you got it in $count tries\n";
print "seed = $seed  rand = $rand ( last guess = $lastguess )\n";
[download]

Added: As a bonus, here's cleaner code and output:

use strict;
use warnings;

my $low   = 1;
my $high  = 10;
my $range = $high - $low + 1;
my $rand  = $low + int(rand($range));
my $count = 0;

for (;;) {
    my $guess = $low + int(rand($range));
    $count++;
    print("$count) Guessing $guess.");

    last if $guess == $rand;

    if ( $guess < $rand )
    {
        print " Too low.\n";
        $low = $guess + 1;
    }
    elsif ( $guess > $rand )
    {
        print " Too high.\n";
        $high = $guess - 1;
    }

    $range = $high - $low + 1;
}

if ($count == 1) {
   print " Got it on the first try!\n";
} else {
   print " Got it in $count tries.\n";
}
[download]

It's "cleaner" because (1) a list (@range) isn't built where one isn't needed, (2) $count is updated when a guess is actually made, (3) $seed is more appropriately called $guess, and (4) $seed isn't checked against $guess until a guess is actually made. Also, (5) I removed the previous guess from the output, since (A) it would need to be omitted on the first guess, and (B) the previous guess is already displayed on the previous line.