First of all: please use code tags around code samples, it is a wonder that your code is actually displayed correctly.

The following is at least working code, I'm not sure that it does anything you wanted. I changed the to-be-substituted character to something more visible

perl -e '$mystring="CONTENT- TYPE\n"; $mystring=~s/(.*)/{my $foo= $1; 
+$foo=~s\/N\/ \/; "$foo"}/e; print $mystring;'
#prints
CO TENT- TYPE
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Note that to substitute \n in this example you would have to make sure that the '.' pattern recognises \n with a regex switch 's':

perl -e '$mystring="CONTENT- TYPE\n"; $mystring=~s/(.*)/{my $foo= $1; 
+$foo=~s\/\n\/ \/; "$foo"}/se; print $mystring;'
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Hi, i have a need to change matches from a regex, to be changed on the fly, in the replace section.

That is as clear as mud.

use strict;
use warnings;
use 5.010;


my $string = 'a6a';
my $new_val = 9;

$string =~ s/(\d+)/
            $1 + $new_val
            /e;

say $string;

--output:--
a15a



$string =~ s/(\d+)/
            "hello$1world"
            /e;

say $string;

--output:--
ahello15worlda


$string = 'aXXXa';

$string =~ s/(XXX)/
            my $x = $1;      #$1 is read only, so can't use s/// on it
            $x =~ s{X}{Y}g;  #use a unique delimiter so no conflict
            $x               #previous line returns # of substitutions
            /e;
say $string;

--output:--
aYYYa
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