Fix Perl errors

"my" variable $cmd masks earlier declaration in same scope at /tmp/contrib_collectd2html.pl line 223.
syntax error at /tmp/contrib_collectd2html.pl line 219, near "$span qw(1hour 1day 1week 1month)"
Global symbol "$span" requires explicit package name at /tmp/contrib_collectd2html.pl line 221.
Global symbol "$span" requires explicit package name at /tmp/contrib_collectd2html.pl line 224.
Global symbol "$span" requires explicit package name at /tmp/contrib_collectd2html.pl line 225.
Global symbol "$span" requires explicit package name at /tmp/contrib_collectd2html.pl line 232.
Global symbol "$span" requires explicit package name at /tmp/contrib_collectd2html.pl line 237.
syntax error at /tmp/contrib_collectd2html.pl line 245, near "}"