=head1 NAME

FileHandle::Sub - an output filehandle that sends each line of output 
+to a user-specified CODE block

=head1 SYNOPSIS

    use FileHandle::Sub;

    # grep
    my $fh = FileHandle::Sub::open { /token/ and print };

    # prefix
    my $fh = FileHandle::Sub::open { s/^/scalar localtime/e; print };

=head1 DESCRIPTION

Each line of output sent to this file handle will be passed to the COD
+E block supplied to C<open>.

=cut

package FileHandle::Sub;
{
    require 5.8.0;

    use strict;
    use warnings;

    sub open (&)
    {
        local *FH;
        tie *FH, __PACKAGE__, @_ or return;
        return *FH;
    }

    sub TIEHANDLE
    {
        my($class, $code) = @_;
        bless [$code, ""], $class;
    }

    sub _emit
    {
        my ($self, $txt) = @_;

        my ($code, $prev) = @$self;

        if ($txt =~ /\n$/)
        {
            local $_ = $prev . $txt;
            $code->($_);
            $prev = "";
        }
        else
        {
            $prev .= $txt;
        }

        $self->[1] = $prev;
    }

    sub PRINT
    {
        my ($self, @txt) = @_;
        local $_;
        for my $txt (@txt)
        {
            _emit($self, $_) for $txt =~ /[^\n]*\n?/g;
        }
    }

    sub PRINTF
    {
        my ($self, $fmt, @args) = @_;
        PRINT $self, sprintf $fmt, @args;
    }

    sub CLOSE
    {
        my ($self) = @_;
        local $_ = $self->[1];
        if (length)
        {
            $self->[0]->($_);
        }
        $self->[1] = "";
    }

    sub DESTROY { &CLOSE }
}

1;
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