First, there is no module Perl::mecha. It helps people to give you better answers if you put some effort into the title of your question. For example, a better title would have included the real module name in it.

Second, in what sense do you mean "seems to overload the machine"? What symptoms did you witness that make you conclude "overloading of the machine"?

Third, what steps have you taken to ensure that the problem is at all related to WWW::Mechanize::Firefox at all? If you don't know what a Perl warning means, use the diagnostics pragma to get more explanation:

print() on unopened filehandle FH at -e line 1 (#2)
    (W unopened) An I/O operation was attempted on a filehandle that w
+as
    never initialized.  You need to do an open(), a sysopen(), or a so
+cket()
    call, or call a constructor from the FileHandle package
[download]

Alternatively, print() on closed filehandle OUTPUT also gives lots of answers that will tell you that you did not use autodie and also did not check the return value of open.