sub fixedwidth_linebreaks
{
    my( $str, $width ) = @_;
    my $newstr;
    while ( $str )
    {
        $newstr .= substr( $str, 0, $width, '' )."\n";
    }
    $newstr
}

$str = fixedwidth_linebreaks( $str, 128 );
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Note that this adds a newline onto the last segment, even if it's shorter than the specified width.