#!perl=C:\bin\perl
use warnings;
use strict;

use Tie::File;

tie my @data, 'Tie::File', 'myfile.txt' or die $!;
sub inc { 
    my ($num) = @_; ++$num 
} 
s/(MyString\d+)/$1 . (inc($2))/eg for @data;

untie @data;