use utf8;
use strict;
use warnings FATAL => 'all';
use Data::Munge qw(list2re);
use File::Slurp qw(read_file write_file);

my %tr = (
    a => 'а',
    b => 'б',
    c => 'ц',
    A => 'А',
    B => 'Б',
    C => 'Ц',
);
my $key = list2re keys %tr;

my $text = read_file('test.txt', { binmode => ':encoding(UTF-8)' });

$text =~ s{
    @       # fragment start
    ([^@]+) # capture characters inside (all except @)
    @       # fragment end
}{
    my $fragment = $1;
    $fragment =~ s{
        ($key)
    }{
        $tr{$1}
    }egmsx;
    $fragment;
}egmsx;

write_file('output.txt', { binmode => ':encoding(UTF-8)' }, $text);