hello gimcdan,
aside from pack, the mention of pixel let me think you are inspecting a template for an image: my little experience with bitmap was due to my project about Tartaglia's triangle.

As far as i remember in the image you have some header where you declare the size of the bitmap (x and y), the number of colors used and a fourth parameter i dont know..
Then you can declare a map between char and colors. In the example below the space is the background (mapped to black) and the # is mapped to red.This is mentioned in Mastering PerlTk also.

So, if you are talking about bitmap, any char is acceptable to set a pixel because it sets it's color.

sub tart_icon {
    return <<EOI
/* XPM */
static char * Icon_xpm[] = {
"32 32 4 1",
"     c #000000000000",
"g    c #00FF00",
"X    c #FF0000",
"D    c #FFFF00",
"                                ",
"                                ",
"                                ",
"                                ",
"                                ",
"                                ",
"        XXXXXXXXXXXXXXXX        ",
"        XXXXXXXXXXXXXXXX        ",
"                                ",
"                                ",
"               XX               ",
"               XX               ",
"              XXXX              ",
"              XXXX              ",
"             XXXXXX             ",
"             XXXXXX             ",
"            XXXXXXXX            ",
"            XXXXXXXX            ",
"           XXXXXXXXXX           ",
"           XXXXXXXXXX           ",
"          XXXXXXXXXXXX          ",
"          XXXXXXXXXXXX          ",
"         XXXXXXXXXXXXXX         ",
"         XXXXXXXXXXXXXX         ",
"                                ",
"                                ",
"                                ",
"                                ",
"                                ",
"                                ",
"                                ",
"                                ",};
EOI
}
[download]

HtH
L*