Nice.

This seems to be the same idea as the UNIX tool split. Which, by the way, as a pure Perl implementation in the Perl Power Tools project.

Um... This seems rather awkward:

if($chunk_size=~/(\d+)([mMgG])/) {
    my $numb=$1;
    my $tp=$2;
    SW: {
        ($tp eq "g" || $tp eq "G") && do {
            $numb*=1024*1024*1024;
            $size=$numb;
            last SW;
        };
        ($tp eq "m" || $tp eq "M") && do {
            $numb*=1024*1024;
            $size=$numb;
            last SW;
        };
    }
}
else {
    $size=$chunk_size;
}
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I think the following would be bit more natural:

if ( $chunk_size =~ /(\d+)g/i )
{
    $size = $1 * 1024*1024*1024;
}
elsif ( $chunk_size =~ /(\d+)m/i )
{
    $size = $1 * 1024*1024;
}
else
{
    $size = $chunk_size;
}
[download]

Of course, it could be further succinctified:

$size = $chunk_size =~ /(\d+)g/i ? $1 * 1024*1024*1024 :
        $chunk_size =~ /(\d+)m/i ? $1 * 1024*1024 :
        $chunk_size =~ /(\d+)k/i ? $1 * 1024 :          # easy additio
+n
        $chunk_size;
[download]