if ($output = `firefox`)
    {print "no browser found\n"}
[download]

would mean that you invoke firefox, wait for firefox to exit and stuff its output into $output. Meanwhile (while firefox is running) your application is blocked. Is that the intended behaviour?

What happens if the invocation fails? Most probably the shell will send an error message to its STDERR, which you don't collect in $output. Bummer, no error message "no browser found"...

Collecting the output of a program that doesn't exist is no good to test its existence. A better approach would be

BROWSER: {
    for my $dir (split /:/, $ENV{PATH}) {
        for my $browser (qw(firefox mozilla opera konqueror)) {
            if (-x "$dir/$browser") {
                system "$dir/$browser"   # or "$dir/$browser &" to bac
+kground
                    and die "running $browser returned $?\n";
                last BROWSER;
            }
        }
    }
    print "no browser found.\n";
}
[download]

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Actually, I meant

 
    if (`firefox&`)
    {print "no browser found\n"}
[download]

but I wrote it wrong in the previous post! I should better pay attention next time I start hearing "Brain overflow...out of memory"

That's bogus, and you are still not getting STDERR:

if(`nonexisting &`) {
    die "running 'nonexisting' produced whatever output.\n"
}
print "all well.\n";
__END__
sh: nonexisting: command not found
all well.
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}