Perl 5 version 8.9 documentation
Returns the offset of where the last
m//gsearch left off for the variable in question (
$_is used when the variable is not specified). Note that 0 is a valid match offset.
undefindicates that the search position is reset (usually due to match failure, but can also be because no match has yet been performed on the scalar).
posdirectly accesses the location used by the regexp engine to store the offset, so assigning to
poswill change that offset, and so will also influence the
\Gzero-width assertion in regular expressions. Because a failed
m//gcmatch doesn't reset the offset, the return from
poswon't change either in this case. See perlre and perlop.