Try adding a couple of print statements to your program and see what it is doing. 

#! perl -slw
use strict;

my @numbers = (3, 2, 1);
my $highest;

## NOTE Perl arrays start from 0 (zero) not 1 (one).
for (my $i = 0; $i < @numbers; $i++)   {
    print "Comparing $numbers[$i] with $numbers[$i+1]";
    if ($numbers[$i] > $numbers[$i + 1])   {
        print "Setting \$highest to $numbers[$i]";
        $highest = $numbers[$i];
    }
 }

 print $highest;
__END__

[download]

prints 

C:\test>217736
Comparing 3 with 2
Setting $highest to 3
Comparing 2 with 1
Setting $highest to 2
Use of uninitialized value in concatenation (.) or string at C:\test\2
+17736.pl line 8.
Comparing 1 with
Use of uninitialized value in numeric gt (>) at C:\test\217736.pl line
+ 9.
Setting $highest to 1
1

[download]

Does that make it any clearer for you?

Okay you lot, get your wings on the left, halos on the right. It's one size fits all, and "No!", you can't have a different color.
Pick up your cloud down the end and "Yes" if you get allocated a grey one they are a bit damp under foot, but someone has to get them.
Get used to the wings fast cos its an 8 hour day...unless the Govenor calls for a cyclone or hurricane, in which case 16 hour shifts are mandatory.
Just be grateful that you arrived just as the tornado season finished. Them buggers are real work.

In reply to Re: finding highest number by BrowserUk
in thread finding highest number by Anonymous Monk

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