1. perldoc perlretut (and company)
2. Tutorials
3. "Mastering Regular Expressions," Friedl (O'Reilly)

which is a semi-nice way of saying "RTFM."

Really!

Once you learn to fish in Perl's documentation pond, you'll never go hungry.

I'm sorry, but how that can help me?

I've read 1 and 3 points of your list, and just take a look at second point. They don't contain answers on my questions, which is:

1. Is it possible to set pos() value from regexp itself, and how to do it?
2. How can \G assertion help me to change pos() value?

Look like you have misunderstand me.

perldoc -f pos
       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.  "und
+ef"
               indicates that the search position is reset (usually du
+e to
               match failure, but can also be because no match has yet
+ been
               performed on the scalar). "pos" directly accesses the l
+ocation
               used by the regexp engine to store the offset, so assig
+ning to
               "pos" will change that offset, and so will also influen
+ce the
               "\G" zero-width assertion in regular expressions. Becau
+se a
               failed "m//gc" match doesn't reset the offset, the retu
+rn from
               "pos" won't change either in this case.  See perlre and
+ perlop.
[download]

Point is “"pos" directly accesses the location used by the regexp engine to store the offset” and I want to change it.
Second point is

perl -e "\$_='qwerty';s/r/print pos()/e;"
[download]

so I can read pos value, and it is logical that I can set it too. But I don't know how to do it, and can't find any examples.

Now about \G which says regexp to start next search from the end of previous. It's main purpose to check several regexp on string, special for that its used with c modifier practically all the time. But in my case all actions make in function which called by regexp. But after then function change string, new search must start from earlier position then pos() value. And I don't understand how can \G help me?
Of course I can miss something, but what?
I'm sorry for my language level, i can't says clearly what exactly I mean. But hope you will understand.

I wonder if perhaps you're asking about something like this:

#!/usr/bin/perl
use strict;
use warnings;
# 864348

$_='qwerty erk';
my $pos;

my $foo = $_ =~ s/r/
        $pos=pos(). "\n";
        print pos(); 
        print "\n"; 
        ++( pos() );
        print "\t" . pos() . "\n";
        print($pos . "\n")/gex;

print "Now, outside the regex, $pos";
[download]

Output:

3
        4
3

8
        9
8

Now, outside the regex, 8
[download]

(Note: as you wrote the one-liner in your previous, you wrote an illegal ref to $_; the $ does NOT need escaping in your one-liner.
... and I'm confused: "G" and "g" come and go in your posts... sometimes one; sometimes the other; and sometimes, not at all.)