sub Sec2String
{
    use integer;
    my $sec = shift;
    return "$sec" if $sec < 60;

    my $min = $sec / 60, $sec %= 60;
    $sec = "0$sec" if $sec < 10;
    return "$min:$sec" if $min < 60;

    my $hrs = $min / 60, $min %= 60;
    $min = "0$min" if $min < 10;
    return "$hrs:$min:$sec" if $hrs < 24

    my $days = $hrs / 24, $hrs %= 24;
    $hrs = "0$hrs" if $hrs < 10;
    return "$days:$hrs:$min:$sec";
}
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Although personally I would not bother with the first three return statements.