One character less:

perl -nlE'sub w{[sort/./g]}INIT{$w=w$_=pop}$w~~w&&say' ./dict.txt acer
+t
[download]

And if you don't mind the (second) argument getting printed, another 3 chars less:

perl -nlE'sub w{[sort/./g]}$w||=w$_=pop;$w~~w&&say' ./dict.txt acert
[download]