foreach (@file)
{
    open input file  or die...

    my $part=1;
    
    my $sizeRead;
    
    while ($sizeRead = read $in_fh, $buffer, $chunk;)
    {
       open out file for the current part number
       increment part number for next time if there is such
       write the data...using $sizeRead (what was actually
       read)
    }
    
    if $sizeRead is undef, last read didn't "work".   
}