You can use ShellExecute to use the default app to open any document (including a URL). When you use it with a URL, it'll respect any and all of the settings you have for your default browser, including reusing windows.

Here's an example using Win32::API and ShellExecute.

#!/usr/bin/perl -w

use strict;
use warnings;
use Win32::API;

my $ShellExecute =
  new Win32::API( "shell32", "ShellExecute", [qw(N P P P P N)], 'N' );
my $GetDesktopWindow = 
  new Win32::API( "user32", "GetDesktopWindow", [], 'N' );

my $hWnd = $GetDesktopWindow->Call();
my $url  = 'http://perlmonks.org';

$ShellExecute->Call( $hWnd, 'open', $url, '', '', 1 );
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