use strict;
use warnings;

my %replaces =
  (
   "foo" => {
             'W100' => 'W102',
             'W101' => 'W103',
            },
   "bar" => {
             'W120' => 'W99',
             'W121' => 'W104',
            },
  );


my $expr = join '|', map {keys %$_} values %replaces;

print $expr;