package ShownameFilter;
use Filter::Simple;

FILTER_ONLY code => sub {
    s{
    showname\(
      (                # Capture 2 (closes second)
          \s*
          \\?          # References
          [\$@%*]+     # Scalar, array, hash, glob
          (            # Capture 1 (closes first)
           \w*         # alphanumeric. Doesn't catch a lot of perl special vars
          )            # End capture 1
          \s*
      )                # End capture 2
      \)
    }
    {showname('$2',$1)}xg;
};

1;  

##</code><code>##

use strict;
use warnings;
use ShownameFilter;

sub showname {
    my ($name,$value) = @_;
    print "name: $name\n";
}

my ($x, $x12, @foo, %bar);

showname($x);
showname( $x12 );
showname( \$x12 );
showname( @foo );

##</code><code>##

name: x
name: x12
name: x12
name: foo