$_ = "04abcdefgh";
my $result;

if (/ (\d\d) /gx) {
    $result .= $&;
    my $count = 0 + $1;
    
    if (/ \G .{$count} /x) { $result .= $&; }
    else { $result = undef }
}

dd $result;  #-> "04abcd"

##</code><code>##

"04abcdefgh" =~ / (\d\d)
                  (??{ ". {".(0 + $^N)."}" }) /x;

dd $&;  #-> "04abcd"

##</code><code>##

"04abcdefgh" =~ / (\d\d)
                  (?>
                    (?{ $^N })                # initialize $^R to $1
                    ( .
                      (?(?{ --$^R }) (?-1))   # recurse if --$^R > 0
                    )
                  ) /x;

dd $&;  #-> "04abcd"

##</code><code>##

"04abcdefgh" ~~ / (\d\d) . ** { $0 } /;

##</code><code>##

"04abcdefgh" ~~ / (\d\d) . ** { $/[*-1] } /;

##</code><code>##

"04abcdefgh" =~ / (\d\d) .{ (?{ $1 }) } /x;

##</code><code>##

"04abcdefgh" =~ / (\d\d) .{ (?{ $^N }) } /x;

##</code><code>##

":aa2bb4cc6dd8" =~ / (:)
                     (?: (\w\w) (\d) )* /x;

dd $&;  #-> ":aa2bb4cc6dd8"
dd $1;  #-> ":"
dd $2;  #-> "dd"
dd $3;  #-> 8

##</code><code>##

$_ = ":aa2bb4cc6dd8";
my @result;

if (/ : /gx) {
    $result[0] .= $&;
    
    while (/ \G (\w\w) (\d) /gx) {
        $result[0] .= $&;
        push @{$result[1]}, $1;
        push @{$result[2]}, $2;
    }
}

dd $result[0];  #-> ":aa2bb4cc6dd8"
dd $result[1];  #-> ["aa", "bb", "cc", "dd"]
dd $result[2];  #-> [2, 4, 6, 8]

##</code><code>##

":aa2bb4cc6dd8" =~ / (:)
                     (?{ [[], []] })   # initialize $^R
                     (?:
                         (\w\w) (\d)
                         
                         # add captures to $^R:
                         (?{ [[@{$^R->[0]}, $2], [@{$^R->[1]}, $3]] })
                     )*
                   /x;

dd $&;       #->  ":aa2bb4cc6dd8"
dd $1;       #->  ":"
dd $^R->[0]; #->  ["aa", "bb", "cc", "dd"]
dd $^R->[1]; #->  [2, 4, 6, 8] }

##</code><code>##

":aa2bb4cc6dd8" =~ / (:)
                     (?{ [] })   # initialize $^R
                     (?:
                         (\w\w) (\d)
                         
                         # add captures to $^R:
                         (?{ [@{$^R}, [$2, $3]] })
                     )*
                   /x;

dd $&;   #->  ":aa2bb4cc6dd8"
dd $1;   #->  ":"
dd $^R;  #->  [["aa", 2], ["bb", 4], ["cc", 6], ["dd", 8]]

##</code><code>##

":aa2bb4cc6dd8" ~~ / (":")
                     [ (\w\w) (\d) ]* /;

dd $0.Str;   #-> ":"
dd $1».Str;  #-> ("aa", "bb", "cc", "dd")
dd $2».Int;  #-> (2, 4, 6, 8)

##</code><code>##

use re 'multi_captures';

":aa2bb4cc6dd8" =~ / (:)
                     (?: (\w\w) (\d) )* /x;

dd $&;  #-> ":aa2bb4cc6dd8"
dd $1;  #-> ":"
dd $2;  #-> ["aa", "bb", "cc", "dd"]
dd $3;  #-> [2, 4, 6, 8]