#!/usr/bin/perl

use strict;
use warnings;

$ENV{PATH}="/usr/bin:/usr/local/bin:/bin";
system("firefox http://www.perlmonks.org");
[download]

If you want a default browser, and its under your control you could have users set an environmental var like export BROWSER='/usr/bin/firefox' Other than that, certain distros may have some facility for getting at that, but it would most likely be distro specific. Your mileage may vary.

Untested, but you could also try something such as

#!/usr/bin/perl
#
use strict;
use warnings;

$ENV{PATH}="/usr/bin:/usr/local/bin:/bin";

my @browsers = qq|firefox lynx|;

USEBROWSER: {
    for (@browsers) {
        my $status = (system("$_ http://www.perlmonks.org") == 0 ) ? 0
+ : 1;
        last USEBROWSER if $status == 0;
    }

    print "No suitable browser was found...\n";
}
[download]