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

open (LOGFILE, ">logfile") || die "could not open logfile";

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

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

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

close LOGFILE;