I think you should have to use the ternery operator here: 
my @lexlist = @_ ? @_ : sort keys %lexicon;

[download]
Update - contrary to glancing, || as well as or will not work in this example. By using || or or with parentheses, sort keys %lexicon is evaluated in scalar context. Here is some code to demonstrate: 
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;
}

[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to (jeffa) Re: @_ still mystifies me by jeffa
in thread @_ still mystifies me by PetaMem

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.