51 characters:

@;="";$x=0,@;=map{$_.$x,$_.($x^=1)}@;for 1..pop;\@;
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Am I going to get myself in trouble using random punctuation mark variables like @;? It saved me one space character...

Update: Down to 45, even passes strict and warnings!

[map{sprintf"%0$_[0]b",$_^$_/2}0..2**$_[0]-1]
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