All right, here's a new version. This one has support for filtering and unfiltering filehandles by calling functions on them, as opposed to the object going out of scope method.

Usage below.

    package Filter::Handle;
    use strict;

    sub import {
        my $class  = shift;
        return if !@_;

        my $caller = caller;
        if ($_[0] eq "subs") {
            no strict 'refs';
            for my $sub (qw/Filter UnFilter/) {
                *{"${caller}::$sub"} = \&{"${class}::$sub"};
            }
        }
    }

    sub Filter {
        my $fh = $_[0];
        tie *{ $fh }, __PACKAGE__, @_;
    }

    sub UnFilter {
        my $fh = shift;
        { local $^W = 0; untie *{ $fh } }
    }

    sub TIEHANDLE {
        my $class = shift;
        my $fh       = shift or die "Need a filehandle.";
        my $output   = shift || sub {
            my($file, $line) = (caller(1))[1,2];
            sprintf "%s:%d - %s\n", $file, $line, "@_"
        };
        bless { fh => $fh, output => $output }, $class;
    }

    sub new {
        Filter(@_[1..$#_]);
        bless { fh => $_[1] }, $_[0]
    }

    sub DESTROY {
        my $self = shift;
        UnFilter($self->{fh});
    }

    sub PRINT {
        my $self = shift;
        my $fh = *{ $self->{fh} };   ## thanks, chromatic :)
        print $fh $self->{output}->(@_);
    }

    sub PRINTF {
        my $self = shift;
        my $fmt   = shift;
        @_ = ($self, sprintf $fmt, @_);
        goto &PRINT;    ## thanks, tilly :)
    }

    1;
[download]

Usage is either what we had before:

    my $f = Filter::Handle->new(\*STDOUT,
        sub { "Foo: @_\n" });
    print "Bar";
[download]

Or the new

    use Filter::Handle qw/subs/;
    Filter \*STDOUT, sub { "Foo: @_\n" };
    print "Bar";
    UnFilter \*STDOUT;
[download]

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