use strict;
use warnings;
use feature 'say';

my $str = 'This ia a real number, 123456.56';
say $str;

(my $res = $str) =~ s{ (\d+) }{ sprintf("%s", 4*$1) }xe; # $1*4, just to show /e works

say $res;

say "\$1 = $1";
(my $adj = $1) =~ s/(\d)/3/g;  # replace each \d with '3', same reason
say $adj;

($res = $str) =~ s{ (\d+) }{ ($adj = $1) =~ s/(\d)/3/g; $adj }xe; # replace '123456' with '333333', proof of concept

say $res;