sub convert {
      my %arg = @_;
      my ($from, $to, $thing) = @arg{qw /from to thing/};
      ...
   }

##</code><code>##

  sub convert {
      my %arg   = @_;
      my $from  = exist $arg{from}  ? $arg{from}  : 'default';
      my $to    = exist $arg{to}    ? $arg{to}    : 'default';
      my $thing = exist $arg{thing} ? $arg{thing} : 'default';
      ...
}

##</code><code>##

  sub convert {
      my %arg = (from   => 'default',
                 to     => 'default',
                 thing  => 'default',
                 @_);
      my ($from, $to, $thing) = @arg{qw /from to thing/};
      ...
   }

##</code><code>##

  sub convert {
      my %arg   = @_;
      my $from  = $arg{from}  || 'default';
      my $to    = $arg{to}    || 'default';
      my $thing = $arg{thing} || 'default';
      ...
   }

##</code><code>##

  sub convert {
      local @ARGV = @_;
      GetOptions ('from:s'  => \my $from,
                  'to:s'    => \my $to,
                  'thing:s' => \my $thing);
      ...
  }