Quick golf trick: push() has a return value :)
Replace push@a,/./g;$r||=@a with $r||=push@a,/./g and save 3 strokes.
UPDATE: Oops. that what I get for not testing :(

Alternates: 

push@a,@r=/./g}{print$s{-1+map{$u{$_}||=$a[$_]+$s{$g-1}for
grep$a[$_]*!$s{$_},$g+@r,$g-!!($g%@r),$g-@r*($g>=@r),++$g*!!($g
%@r);($g)=sort{$u{$a}-$u{$b}}keys%u;$s{$g}=delete$u{$g}}@a}

[download]

and 

# problem description says "resembles a square"
push@a,/./g}{print$s{-1+map{$u{$_}||=$a[$_]+$s{$g-1}for
grep$a[$_]*!$s{$_},$g+$.,$g-!!($g%$.),$g-$.*($g>=$.),++$g*!!($g
%$.);($g)=sort{$u{$a}-$u{$b}}keys%u;$s{$g}=delete$u{$g}}@a}

[download]

In reply to Re: Advent of Code, Day 15, golfed! by tybalt89
in thread Advent of Code, Day 15, golfed! by Jasper

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