c:\@Work\Perl\monks>perl -wMstrict -le
"use feature 'state';
 ;;
 my $s = '1 {\d1} 2 {\d2} xx{\d3}yy';
 print qq{'$s'};
 ;;
 $s =~ s{ \{ \\d (\d+) \} }{ ran_digs($1) }xmsge;
 print qq{'$s'};
 ;;
 sub ran_digs {
   state $powers = { map { $_ => 10 ** $_ } 1 .. 7 };
   ;;
   return int rand $powers->{$_[0]};
   }
"
'1 {\d1} 2 {\d2} xx{\d3}yy'
'1 7 2 18 xx406yy'