Yes, the -w switch is acceptable - there was in fact no warnings pragma before Perl 5.6, so it's a fairly recent thing. I still prefer -w for the time being as I never need to switch off warnings anywhere, anyway. (The warnings pragma allows for a more finegrained control if you do: you don't have to switch warnings off entirely anymore, you can selectively disable only specific ones.)

The script is fine, "clean" wise, but could be written quite a lot more succintly. The only technical mistake is writing "$WH" etc where $WH (without the quotes) would do. If you're only using a single variable, you almost never want to put it in quotes. (There are a rare few cases, but you'll know those when you see them.)

One thing I strongly urge you to, though, is to properly indent your code. It is barely acceptable in that short script, but would make a more complex one completely unreadable.

Another note is that you're outputting a CGI header in your printGIF routine: if you go to the trouble of writing a function, then make it do exactly one thing. In this case, generate a GIF file. Nothing else. The header, in this case, is the main program's job. That way, you get functions you can reuse in other scripts later. For the same reason I would have the function return the GIF file, rather than printing it to STDOUT directly.

Which brings up another point you might want to know about: the $|++; I dumped in there. See Suffering from Buffering about it.

Here's how I'd write that:

#!/usr/bin/perl -w
# this script outputs a gif 
# set color, width, and height
use strict;

$|++;

binmode STDOUT;

print "Content-Type: image/gif\n\n";
print gif_file "FF0000", 0x33, 0x10;

######################################
sub gif_file{
    my ($hexrgb, $wid, $hgh) = @_;

    my %c;
    @c{qw(r g b)} = map hex, unpack "A2"x3, $hexrgb;

    return pack "C*", (
        0x47, 0x49, 0x46, 0x38, 0x37,  0x61,  $wid,  0x00,
        $hgh, 0x00, 0xA1, 0x01, 0x00, $c{r}, $c{g}, $c{b},
        0xFF, 0xFF, 0xFF, 0x00, 0x00,  0x00,  0x00,  0x00,
        0x00, 0x21, 0xF9, 0x04, 0x05,  0x00,  0x00,  0x01,
        0x00, 0x2C, 0x00, 0x00, 0x00,  0x00,  $wid,  0x00,
        $hgh, 0x00, 0x40, 0x02, $hgh,  0x84,  0x8F,  0xA9,
        0xCB, 0xED, 0x0F, 0xA3, 0x9C,  0xB4,  0xDA,  0x8B,
        0xB3, 0xDE, 0x9C, 0x17, 0x00,  0x3B,
    );
}
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