use strict;
use Data::Dumper;

my %lexicon = (
   foo => 'bar',
   baz => 'qux',
);

foo();
foo(qw(one two three));

bar();
bar(qw(one two three));

baz();
baz(qw(one two three));

sub foo {
   my @lexlist = @_ ? @_ : sort keys %lexicon; # correct
   print "foo:\n", Dumper \@lexlist;
}

sub bar {
   my @lexlist = @_ || sort keys %lexicon;     # incorrect
   print "bar:\n", Dumper \@lexlist;
}

sub baz {
   my @lexlist = (@_ or sort keys %lexicon);   # incorrect
   print "baz:\n", Dumper \@lexlist;
}