XML::Simple (or any of the XML:: family modules) will treat xml files as text. There is no attempt to evaluate perl expressions within that text.

You may be able to handle this as a special case with eval,

# notional dereference, use whatever is correct for your xml struct
$xml->{'Level'} = eval $xml->{'Level'};
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Constants from constant.pm are really subs with an empty prototype, so you may need to append '()' or even write ${\LOG_DEBUG()} to get the evaluation done in a quoted string.

In this case, it might be simpler to do a substitution.

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;
}
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