Something you haven't considered is backtracking. Your code, cleaned up:

my @strings = qw( acaabbbb );

for ( @strings )
{
    local our $a_counter = 0;
    local our $b_counter = 0;
    /
        (?: a (?{ $a_counter++ }) )+
        (?: b (?{ $b_counter++ }) )+
    /x;
    print("a: $a_counter\n");  # a: 3
    print("b: $b_counter\n");  # b: 4
}
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If you want 2,4 instead of 3,4, you need to take backtracking into account.

my @strings = qw( acaabbbb );

for ( @strings )
{
    local our $a_counter = 0;
    local our $b_counter = 0;
    /
        (?{
            # Initialize $^R
            { a => 0, b => 0 }
        })
        (?: a (?{ my %h = %{$^R}; $h{a}++; +{ %h } }) )+
        (?: b (?{ my %h = %{$^R}; $h{b}++; +{ %h } }) )+
        (?{
            # Return results
            my %h = %{$^R};
            $a_counter = $h{a};
            $b_counter = $h{b};
        })
    /x;
    print("a: $a_counter\n");  # a: 2
    print("b: $b_counter\n");  # b: 4
}
[download]