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