my $re = qr/(?<whole_enchilada> (?<Letter>\w+) (?<Digit>\s*\d+\s*)* )/x;

##</code><code>##

#!/usr/bin/env perl
use 5.034;

use YAML::XS qw( Dump );

my @digits;
my $re = qr{(?{@digits=()}) 
            (?<letter>\w+)
            ( (?<digit> \s* \d+ \s*)(?{push @digits, $+{digit}}) )* }x;

my $str = 'a 1 2 3 b 4 5 6';

while( $str =~ m{$re}g ) {
  say qq{$&: '$1'\t}, join( q{|}, @digits );
}

exit 0;

__END__
oscar:~  1027> perl refoo.plx
a 1 2 3 : 'a'    1 |2 |3
b 4 5 6: 'b'     4 |5 |6