I haven't looked at Mechanize's source, but I'm guessing that it doesn't look at <button> elements when parsing the page for forms.

HTML::Form/WWW::Mechanize don't have a problem with buttons inside forms

use WWW::Mechanize 1.75;
my $ua = WWW::Mechanize->new;
$ua->get(q{file:123654789987.html});
$ua->dump_forms;
__END__
POST file:/xxx.cgi?login
  login_name=                    (text)
  password=                      (password)
  login=<UNDEF>                  (submit)
  login=<UNDEF>                  (submit)
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