Close.

my $expr =
   join '|',
      map { keys( %{ $replaces{$_} } ) }
         keys( %replaces );
[download]

With foreach:

my @keys;
for ( keys( %replaces ) ) {
   push @keys, keys( %{ $replaces{$_} } );
}

my $expr = join '|', @keys;
[download]

OP here!
Thanks a lot for your help, I didn't know that map could be used such a way (i.e. having an output instead of altering every item in a list)..
Have a greet day!

That the point of map. It takes a list of inputs, and produces a different list from those inputs. It does so by applying the transformation function (the block or expression) to each input, and collecting the outputs of that function.

Examples:

my @uc = ( uc("abc"), uc("def") );

     |
     v

my @uc = map { uc($_) } "abc", "def";
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my %h = ( "abc" => 1, "def" => 1 );

     |
     v

my %h = map { $_ => 1 } "abc", "def";
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I didn't know that map could be used such a way (i.e. having an output instead of altering every item in a list)

Erm, that is what map is for! :) Perl::Critic even provides a ProhibitVoidMap policy to warn you of code that uses map in void context.

I suggest you follow the simple stylistic advice given in Effective Perl Programming in the item "Use foreach, map and grep as appropriate" namely:

• Use foreach to iterate read-only over each element of a list
• Use map to create a list based on the contents of another list
• Use foreach to modify elements of a list
• Use grep to select elements in a list

Perhaps a bit simpler with values since the second-level hashes you are getting the keys of are the values of the first-level hash. However, if order is important you will have to sort keys in some fashion.

johngg@abouriou:~/perl/Monks$ perl -Mstrict -Mwarnings -E '
my %replaces = (
   foo => { W100 => q{W102}, W101 => q{W103} },
   bar => { W120 => q{W99},  W121 => q{W104} },
   );

my $expr = join q{|}, map { keys %{ $_ } } values %replaces;
say $expr;'
W121|W120|W101|W100
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I hope this is helpful.

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;
[download]

(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.