#! perl
use strict;
use warnings;
use Data::Dump;

my $re  = 'aaa\s+bbb';
my %dic = ($re => 'new');
my $s   = 'aaa   bbb';

$s =~ s/($re)/$dic{$1}/;

print "\$1   = $1\n";
print "\$s   = $s\n";
print "\%dic = ";
dd     \%dic;