Here's the straight-ahead dumb way to do it:

#!/usr/bin/perl -w
use strict;

my $target=shift;

X:
for my $x (1 .. 20) {
    for my $y (1..20) {
        my $z = $target - $x - $y;
        next X if $z<1 || $z>20;
        print "($x,$y,$z)$/";
    }
}
[download]

Uhhh, this wasn't homework, was it? ;-)

Update: Remove a bogosity from code. U ²: samtregar, there is Steal This Code.

After Compline,
Zaxo