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

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

dd $result;  #-> "04abcd"