my %h; # or %params
...
Getopt::Long::GetOptions(
    'infile=s'  => \$h{infile},
    # or more flexible:
    'outfile=s' => sub { $h{$_[0]} = $_[1] },
   # the anonymous sub above will be called with 2 params
   # when --outfile is detected: the key (outfile) and its value
...
}