This will work. There may be a better/faster way.

my $uptime = `uptime`;
if ($uptime =~ /^\s+\S+\s+up\s+(\S+\s+\S+,\s+\S+),/) {
    print $1;
}
else {
    print "uptime gave me a weird format!";
}
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Another way of doing it would be to split the output on \s+, then collect the parts that you want.