Perhaps you meant something like this?

open UP, "< /proc/uptime"
    or  die "Can't read /proc/uptime: $!$/";
my @boot= ( localtime
    time() - ( split ' ', <UP> )[0]
)[ reverse 0..5 ];
close UP;
$boot[0] += 1900;
$boot[1]++;
$_= sprintf "%02d", $_   for  @boot;
my $boot= join( '-', @boot[0..2] )
    . '/' . join( ':', @boot[3..5] );
print "Last booted $boot$/";
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Note that I ignored much of your requested format because I don't think that even you want to report seconds but not minutes and I refuse to use 2-digits years, put colon between date parts, nor order date/time fields other then such that they make sorting easy (and doing all three makes a date pretty unrecognizable) -- that is, I follow the international standard. (: