You can just use an HTML like RGB hex mask, prefixed by '#':

#!/usr/bin/perl
use common::sense;
use PDF::API2;

my $pdf = PDF::API2->new();

my $page = $pdf->page(0);
my $gfx = $page->gfx;

my $red_rgb = '#ff0000';
$gfx->strokecolor($red_rgb);
$gfx->rect(10,100,200,200);
$gfx->stroke;

$pdf->saveas('/tmp/red_box.pdf');
[download]

This'll work pretty well everywhere you can specify a color.