this code does the same: :)

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

Upd or even without a sub:

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

Update: s/\$1/\$_/g in the last onliner