To work on $_ by default, check the size of @_. For example,
sub func { local $_ =   @_ ? $_[0] : $_;  ... } # copy
sub func { local *_ = \(@_ ? $_[0] : $_); ... } # alias

To request a scalar argument, use prototypes. For example,
sub func($)  { ... } # scalar
sub func(;$) { ... } # optional scalar
This is required for $str = my_uc my_reverse 'ab', 'cd'; to work.

Finally, use wantarray to determine whether a list or a scalar (or nothing at all) should be returned.

In Perl, your functions would be the following: 

sub pp_lcfirst(;$) {
   local $_ = @_ ? $_[0] : $_;
   s/(.)/\l$1/;
   return $_;
}

sub pp_uc(;$) {
   local $_ = @_ ? $_[0] : $_;
   s/(.*)/\U$1/;
   return $_;
}

sub pp_reverse {
   if (wantarray) {
      my @rv;
      push(@rv, pop(@_)) while @_;
      return @rv;
   } else {
      my $str = join('', @_);
      my $rv = '';
      my $i = length($str);
      $rv .= substr($str, $i, 1) while $i--;
      return $rv;
   }
}

sub spaces_to_und(;$) {
   local $_ = @_ ? $_[0] : $_;
   s/\s+/_/g;
   return $_;
}

[download]

Update: Cleanup.

Update: Fixed the typo japhy noticed in my prototypes.

In reply to Re: subroutine bewilderment: how to mimic builtins by ikegami
in thread subroutine bewilderment: how to mimic builtins by dimar

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.