1. Printing the line number where the log_error was called without including it in the argument

The four Carp routines and the two Carp::Heavy routines provide this info (using caller).

use Carp ();

sub log_error {
   goto(&Carp::carp) if pop(@_) >= THRESHOLD;
}
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I think Log::Log4perl also prints the file name and the line number. The above and Log::Log4perl don't meet your second requirement, however.

2. I'd like anything < THRESHOLD for the current run to be removed at compile time if possible to avoid a performance hit...not sure if can really do that...or if it makes a difference.

That's not possible if the level is passed as an argument. Your syntax would have to be

sub DEBUG     () { 0 }
sub FATAL     () { 4 }

sub THRESHOLD () { 0 }

log_error($msg) if FATAL >= THRESHOLD;
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Well, either that or use a source filter.

Maybe you could compile out debug messages, but leave the other ones in?

use Carp ();

sub WARN  () { 1 }
sub ERROR () { 2 }

sub DEBUG     () { 0     }  # True of false.
sub THRESHOLD () { ERROR }  # Or use $THRESHOLD. No real diff.

sub log_error {
   my $level = (@_==2 ? pop(@_) : THRESHOLD);
   return if $level < THRESHOLD;
   goto &Carp::carp;
}

log_error($msg) if DEBUG;  # Whole statement removed at compile time.
log_error($msg, WARN);     # Calls log_error, but not carp.
log_error($msg, FATAL);    # Calls log_error and carp.
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I can highly recommend Log::Log4perl. It matches your requirements very closely. I use it daily and it's terrific.