$txt="Arkarmit ithaaatokkk ide? aaaztamindenit. tokkkal-vonoval";

# megkeressuk a szukseges mintat
$txt =~ /aaa(\w{5})/;
# Ami a $1-be kerul. Onnan atmentjuk $text_to_replace -be
$text_to_replace=$1;

$txt =~ s/$text_to_replace/REPLACED/g;
print($txt);