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sub catch_exons2 {
    my @res;
    push @res, [substr($1, 0, 1), length $1] while $_[0] =~ /(a+|x+)/g
+;
    return @res;
}

$string = 'aaaaxxxaxaxaaaaaaaxxxxxxxxxxaax';
@regions = map { [substr($_, 0, 1), length $_] } $string =~ /(a+|x+)/g
+;