Modelled using tye's date format, but using the very handy strftime from POSIX to avoid date math and string assembly:

use POSIX qw(strftime);

sub uptime {
    local $/ = " ";
    open my $fh, '<', '/proc/uptime'
        or die "Can't open /proc/uptime to read: $!\n";
    return scalar <$fh>;
}

print strftime "Last booted %Y-%m-%d/%H:%M:%S\n", (
    localtime( time() - uptime() )
);
[download]

I originally wrote it using an $uptime temporary with a do block initializer, then noticed I can basically switch to a sub by substituting sub uptime for my $uptime = do. Either way makes the code self-documenting.