From perldoc -f pos:

     pos SCALAR

     pos

     Returns the offset of where the last C<m//g> search
     left off for the variable is in question (C<$_> is
     used when the variable is not specified).  May be
     modified to change that offset.  Such modification
     will also influence the C<\G> zero-width assertion in
     regular expressions.  See L<perlre> and L<perlop>.
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If you want the position in the string where the match *starts* (rather than where it left off, as pos gives you), you could use

    pos() - length $&
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or something. For example:

    my $R = "foo bar quux baz";
    while ($R =~ /ba.\b/g) {
        print pos($R) - length $&, "\n";
    }
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Prints:

    4
    13
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