system 'start', $htmlfile and die "could not invoke browser: $!\n";
[download]

and it will open the default browser. On some linux distributions, there is htmlview which will open whatever browser is installed / configured in the preferences:

system "htmlview '$htmlfile' &" and die "starting htmlview failed: $!\
+n";
[download]

Note - system ... and die - 'and' is necessary because system returns nonzero on failure.

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