pos SCALAR
    pos     Returns the offset of where the last "m//g" search left off for
            the variable in question ($_ is used when the variable is not
            specified). Note that 0 is a valid match offset. "undef"
            indicates that the search position is reset (usually due to
            match failure, but can also be because no match has yet been run
            on the scalar).

            "pos" directly accesses the location used by the regexp engine
            to store the offset, so assigning to "pos" will change that
            offset, and so will also influence the "\G" zero-width assertion
            in regular expressions. Both of these effects take place for the
            next match, so you can't affect the position with "pos" during
            the current match, such as in "(?{pos() = 5})" or "s//pos() =
            5/e".

            Setting "pos" also resets the *matched with zero-length* flag,
            described under "Repeated Patterns Matching a Zero-length
            Substring" in perlre.

            Because a failed "m//gc" match doesn't reset the offset, the
            return from "pos" won't change either in this case. See perlre
            and perlop.