Another way which may prove to be other a better alternative is to read the uptime directly from /proc/uptime (assuming that this interface is available to you) - This allows for vagarities of uptime output and regular pattern matching to sidestepped altogether.

For example:

use Date::Calc qw/ Normalize_DHMS /;
use IO::File;

#   get_uptime function - returns uptime from /proc/uptime
#   as 4-member array representing current system uptime in
#   days, hours, minutes and seconds

sub get_uptime {
    my $uptime;
    my $fh = IO::File->new('/proc/uptime', 'r');
    $uptime = (split /\s+/, <$fh>)[0] if defined $fh;
    return Normalize_DHMS(0, 0, 0, $uptime) if defined $uptime;
}
[download]