$txt_original="Arkarmit ithaaatokkk ide? aaaztamindenit. tokkkal-vonoval";
$txt_replaced=$txt_original;
# megkeressuk a szukseges mintat
$txt_replaced =~ /aaa(\w{5})/;
# Ami a $1-be kerul. Onnan atmentjuk $text_to_replace -be
$text_to_replace=$1;
$txt_replaced =~ s/$text_to_replace/12345/g;

print("To replace: ${text_to_replace}\n");
print("Original: ${txt_original}\n");
print("Replaced: ${txt_replaced}\n");

# output:
# -------
#
# To replace: tokkk
# Original: Arkarmit ithaaatokkk ide? aaaztamindenit. tokkkal-vonoval
# Replaced: Arkarmit ithaaa12345 ide? aaaztamindenit. 12345al-vonoval