my $text =~ s[(?<=BEGIN_MARKER)(.*?)(?=END_MARKER)]{
    (my $rep = $1) =~ s[i][x]g; # For single char subs you could use tr// here
    $rep; 
}ge;

print $text;
An abitrary BEGIN_MARKER strxng whxch END_MARKER could contain BEGIN_MARKER just about END_MARKER anything