Look-aheads and look-behinds are IMO advanced constructions we don't need most of the time for simple problems. They demand more thought and many times are not what we wanted at the end. For example, from your description ("I expected it not to match because of the '-' at the end.") and your two solutions, I would suggest the use of simple /\d{4}-\d{1-3}([^-\d]|$)/, where [^-\d] prevents the pattern to match after 1, 2 or 3 digits and encountering yet another digit or dash, and $ in the alternation makes it succeed at the end of the line.

#!/usr/bin/perl -w

use strict ;
use warnings;

for my $d qw(2005-12 2005-100 2005-1- 2005-12- 2005-123- 2005-1000) {
    printf "%-10s: ", $d;
    if ( $d =~ /\d{4}-\d{1,3}([^-\d]|$)/ ) {
      print "yep\n" ;
    } else {
      print "nope\n";
    }
}
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outputs

2005-12   : yep
2005-100  : yep
2005-1-   : nope
2005-12-  : nope
2005-123- : nope
2005-1000 : nope
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