This is exactly the purpose behind using strict. Doing so, you would have found that you are placing the actual image data in $img_data, but are trying to put $image_file into the database. Switch either one of the variables and it should work fine.

update: I figured you could use a small rewrite. Some points I covered:

• I got rid of the useless temporary download file -- a little bit of in-memory manipulation will do just fine.
• I compare the image contents rather than the file size. In your version, you might have gotten false results if the new image size was the same but an actual different image.
• I changed the way the database update is done. I just used DBI's do() method instead of preparing and executing the SQL. Also, I threw in the use of placeholders.
• I added a check to die() if the image fetch fails. We wouldn't want false updates going on now, would we?

#!perl -w

use strict;
use DBI;
use LWP::Simple;

my $img_url = "http://weather.unisys.com/satellite/sat_ir_west.gif";
my $latest_img = "/usr/home/mhearse/weather_images/latest.gif";

my ($min, $hour, $day, $mon, $year) = (localtime())[1..5];
$year += 1900;

my $img_name = "$mon-$day-$year-$hour:$min";
my $img_data = get($img_url);

die("failed to fetch image from internet!\n");
    unless ( defined($img_data) );

open( my $fh_latest, ( -e($latest_img) ? '+<' : '+>' ), $latest_img )
    or die("could not open file: $!");
binmode($fh_latest);

my $latest_data = do { local $/; <$fh_latest> };

if ($latest_data ne $img_data) {
    seek($fh_latest, 0, 0);
    truncate($fh_latest, 0);
    print $fh_latest $img_data;

    my $dbh = DBI->connect(
        'dbi:mysql:weather_images', 'user', 'secret', {
            RaiseError => 1, AutoCommit => 1
        }
    );

    $dbh->do(
        'INSERT INTO gw_ir (image_name, image_file) VALUES(?,?)',
        undef, $img_name, $img_data
    );

    $dbh->disconnect();

    warn("File was not current; updates made.\n");
}

else {
    warn("File was current, no updates made.\n");
}
[download]

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