$ perl -e '
> $str = q{abcXghiXstu};
> @elems = split m{X}, $str;
> print qq{$_\n} for @elems;'
abc
ghi
stu
$ perl -e '
> $str = q{abcXghiXstu};
> @elems = split m{(X)}, $str;
> print qq{$_\n} for @elems;'
abc
X
ghi
X
stu
$

##</code><code>##

$ perl -e '
> $str = q{abc123def456ghi};
> @elems =
>    split m
>    {(?x)
>       (?:
>          (?<=[a-z]) # look behind for letter
>          (?=[0-9])  # look ahead for digit
>       )
>       |             # or
>       (?:
>          (?<=[0-9]) # look behind for digit
>          (?=[a-z])  # look ahead for letter
>       )
>    }, $str;
> print qq{$_\n} for @elems;'
abc
123
def
456
ghi
$