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