gistfile1.pl

#!/usr/bin/env perl

package RangeFile;

use strict;
use warnings;
use 5.10.0;
use Fcntl qw/:seek/;
use parent qw/Exporter/;

our @EXPORT = qw/rangefile/;

sub rangefile {
    my ($fh, $count, $offset) = @_;
    my $self = bless {
        fh => $fh,
        _count => $count,
        _offset => $offset,
    }, __PACKAGE__;

    if ( ! defined $offset ) {
        seek( $fh, 0, SEEK_CUR) or die $!;
        my $cur = tell($fh);
        die $! if $cur < 0;
        $self->{_offset} = $cur;
    }
    if ( ! $count ) {
        seek( $fh, 0, SEEK_END) or die $!;
        my $end = tell($fh);
        die $! if $end < -1;
        $self->{_count} = $end - $offset;
    }
    seek( $fh, $self->{_offset}, SEEK_SET) or die $!;

    $self;
}

sub getline {
    my $self = shift;
    return if defined $self->{_read} && $self->{_read} >= $self->{_count};
    $self->{_read} = 0 if ! exists $self->{_read};

    my $len = ref $/ ? ${$/} : 65536;
    $len = $self->{_count} - $self->{_read} if $len > $self->{_count} - $self->{_read};
    my $read = read( $self->{fh}, my $buf, $len);
    die $! if ! defined $read;

    $self->{_read} += $read;
    $self->{_offset} = $self->{_offset} + $self->{_read};
    return $buf;
}


sub close { 1 };

1;

package main;

use strict;
use warnings;
use 5.10.0;
use Plack::Builder;

builder {
    sub {
        my $env = shift;
        open( my $fh,  __FILE__) or die $!;
        my $range = RangeFile::rangefile( $fh, 30, 100);
        return [200, ['Content-Type'=>'text/plain'], $range];
    }
}