Do it in two steps.

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

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