FWIW: 

>perl -wMstrict -le
"my $s = '123 234 3456789 3210 34567';
 ;;
 my @m = $s =~ m{ \b ((\d) (??{ qr{\d{$^N}} })) \b }xmsg;
 print qq{@m};
 ;;
 my $i = 2;
 @m = $s =~ m{ \b (??{ ++$i;  qr{\d{$i}}; }) \b }xmsg;
 print qq{@m};
"
234 2 3210 3
123 3456789

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In reply to Re^2: Calculating variable {n} in regex match by AnomalousMonk
in thread Calculating variable {n} in regex match by atreyu

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