I came up with this:

my $beg= "";    # Beginning of new string
my $end= "";    # Ending of new string
my $prev= "";   # Previous token
my $str= "'a1b1a2b1b2a3b1b2b3'db1b2";
for my $sub (  split /([ab]\d+)/, $str  ) {
    next   if  "" eq $sub;
    if(  $prev =~ /^a/  &&  $sub =~ /^b/  ) {
        $end .= "($sub)";
    } else {
        $beg .= $sub;
    }
    $prev= $sub;
}
print "$beg$end\n";
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        - tye (but my friends call me "Tye")