Precedence is biting you. You are concatenating @{$_}[0..2] . " " etc, which is not what you intended. Aside from other problems, this evaluates the array slice in scalar context, for the last element, $_->[2], dropping the other two elements.

Solution: Parentheses. For example, to make the first join a function call instead of a list operator:

#!perl
use strict;
use warnings;

my ($mdate,$mts) = @ARGV;
my $recent =    (map { join("-", @{$_}[0..2]) . " " . join ":", @{$_}[
+3..5] }
                 sort { $a->[0] <=> $b->[0] || $a->[1] <=> $b->[1] || 
+$a->[2] <=> $b->[2] }
                 map { [ split "[-: ]" ] } ($mdate,$mts))[0];
print $recent;
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