See the current Perl documentation for pos.
Here is our local, out-dated (pre-5.6) version:
pos - find or set the offset for the last/next m//g search
Returns the offset of where the last
m//g search left off for the variable is in question (
$_ is used when the variable is not specified). May be modified to change that
offset. Such modification will also influence the
\G zero-width assertion in regular expressions. See the perlre manpage and
the perlop manpage.