I already did something like that here

"Perl makes the fun jobs fun
and the boring jobs bearable" - me

sub time_format
{
    my $time = shift;

    my @parts;

    if ($time >= 3600)
    {
        my $hours = int($time / 3600);
        $time -= $hours * 3600;
        push @parts, $hours;
    }

    if ($time >= 60)
    {
        my $minutes = int($time / 60);
        $time -= $minutes * 60;
        push @parts, $minutes;
    }
    else
    {
        push @parts, 0 if @parts;
    }

    push @parts, $time;

    return sprintf join(':', ('%02d') x @parts), @parts;
}
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