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

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

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

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

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