try something like this:

use Symbol 'gensym';
our $log = gensym;
open $log, ">/path/to/logfile" or die $!;

our $DEBUG = 1;

sub add_log;

local $SIG{__WARN__} = sub { &add_log(shift) };
local $SIG{__DIE__} = sub { &add_log(shift); exit 1 };

sub add_log {
   print $log, shift();
   flush $log if $DEBUG;
}

sub DESTROY {
   close $log;
}
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admitted this is more of a rough draft, but the method should be clear from these few lines.