sub format_time{
    my ( $h, $m, $s ) = @_;

    ...

    warn "\$h is now '$h', length " . length $h;
    if ( length( $h ) < 2 ) {
        warn "going to pad with zero";
        $h = "0" . $h; 
    }

    if ( length( $m ) < 2 ) {
        $m = "0" . $m; 
    }

    return $h . $m . $s2;
}