A bit of tuning after reading japhy's Golf advice drops the score to 243

sub assemble {
## Begin Golf
$w=join'',@_;t("",@_);$w}
sub t{my$s=shift;$w=$s if!@_ and length$s<length$w;
return if!@_;my@b=(@_,@_);
map{t($_,@b[0..@_-2])}_($s,shift@b)while(@b>@_)}
sub  _{($a,$b)=@_;$l=length$a;for(0..$l){$x=substr($a,0,$_).$b;
last if substr($x,0,$l)eq$a}$x
## End Golf
}

print assemble qw(GATTACA ATTACA GATT AAGAT CCC)
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