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. 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.