my $res = {
    re1 => { foo => 'foo_new' },
    re2 => { bar => 'bar_new' },
    re3 => { qux => 'bar_new' },
    re4 => { foobar => 'foobar_new' }
};

my @required = qw ( re1 re2 re4 );
my %active_re = map{ each %{$res->{$_}} } @required;
my $match = join '|', sort{ length $b <=> length $a } keys %active_re;
$match = qr/($match)/;


$str = 'foo bar baz foobar';
$str =~ s/$match/$active_re{$1}/g;
print $str;