If you want to be on the (numerically-) safe side, you can do this:

$s=$t % 60; $t=($t-$s)/60;
$m=$t % 60; $t=($t-$m)/60;
$h=$t % 24; $d=($t-$h)/24;
$out=sprintf "%03d:%02d:%02d:%02d\n", $d,$h,$m,$s;
$out=~s/^0+:|00://g;
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Which is using only integers (assuming $t is integral)