# notional dereference, use whatever is correct for your xml struct
$xml->{'Level'} = eval $xml->{'Level'};

##</code><code>##

my %level = {
    LOG_DEBUG => 0,
    LOG_INFO  => 1,
    LOG_WARN  => 2,
    LOG_ERROR => 3,
);
use constant \%level;
# . . .
{
    my $regex = qr/(${\join '|', keys %level})/;
    s/$regex/$level{$1}/g;
}