use warnings;
use strict;
use Carp;
use Filter::Handle qw/subs/;

open (LOGFILE, ">logfile") or die "could not open logfile: $!";

#filter STDERR through an anonymous sub
Filter \*STDERR, sub {local $_ = "@_"; print LOGFILE "Filtered: $_ "; $_};

#need a signal handler to capture warnings from carp and warn which are not captured by Filter::Handle (but by capturing ourselves and printing to STDERR, they do get picked up by Filter::Handle)
$SIG{__WARN__} = sub {local $_ = "@_"; print STDERR $_};

#prints to both STDERR and to LOGFILE
print STDERR "error!\n";
carp "carp!";
warn "warn!";

#STDERR will no longer be filtered through your sub
UnFilter \*STDERR;

print STDERR "not captured to log\n";
warn "this one got away";

close LOGFILE;