A first pass to check the output complies with the rules?

@c='A'..'Z';map{$x.=splice@c,rand@c,1}1..10;($d,$s)=@ARGV;$n=$d+2;sub 
+p{$t="%${n}s\n";printf" $t+$t ".'-'x$n."\n $t\n",@_}sub x{eval"tr[0-9
+][$x]"for@_}map{@a=map{int rand 10**$d}1..2;$a[@a]=$a[0]+$a[1];x(@a);
+p@a}1..$s
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Updates:

176

$x=join'',sort{.5<=>rand}A..Z;$s=pop;$n=2+pop;$t="%${n}s\n";map{@a=map
+{int rand".1e$n"}1,1;$a[@a]+=$a[0]+$a[1];eval"tr[0-9][$x]"for@a;print
+f" $t+$t ".'-'x$n."\n $t\n",@a}1..$s
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167

$x=join'',sort{.5<=>rand}A..Z;$s=pop;$n=1+pop;$t="%${n}s\n";map{$a[2]+
+=$_ for@a=map{int rand".1e$n"}1,1;eval"y/0-9/$x/"for@a;printf" $t+$t 
+".'-'x$n."\n $t\n",@a}1..$s
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165

$x=join'',sort{.5<=>rand}A..Z;$s=pop;$£=1+pop;$t="%$£s\n";map{$a[2]+=$
+_ for@a=map{int rand".1e$£"}1,1;eval"y/0-9/$x/"for@a;printf" $t+$t ".
+'-'x$£."\n $t\n",@a}1..$s
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