A few things I would like to point out. This doesn't actually convert a decimal number, it converts three decimal numbers each less than 255 into a hex sequence(decimal RGB -> Hex?). Also you don't use the modulus operator '%', which does a integer divide and returns the remainder, rather than having $f = $d / 16;$t = $f * 16; $s = $d - $t; you can just say $s = $d % 16;. And the final thing, rather than having a c-style for loop in convert (which you don't actually use) it would seem clearer to write

foreach $num (@dtrip) {
    my $f = int($num / 16);
    ....

But then again why not just handle any size number :) (with arbitrary base)

BEGIN {
my @map = (0..9, 'A'..'Z', 'a'..'z', qw/! @ $ % ^ & * ( ) _/);

sub convert {
    my $num = shift;
    my $base = shift || 16;
    
    die "Base too big: Not enough characters." if $base > $#map;

    my $conv = '';
    my $sign = $num > 0 ? '' : '-';    
    $num = abs $num;

    while ($num != 0) {
        $conv = $map[$num % $base] . $conv;
        $num = int($num / $base);
    }
    return $sign . $conv;
}
}
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