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;
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(similar to ikegami's but DRYer)

There are multiple issues with your code:

• your data structure was buggy since it swaps ( with {
• your my $expr is scoped° to the loop, hence undef outside
• $expr = will be overwritten with each iteration

°) kind of, strict doesn't complain.