I'd do:

  sub convert {
      my %arg = @_;
      my ($from, $to, $thing) = @arg{qw /from to thing/};
      ...
   }
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I don't enforce policies of "even if you want to pass in an argument with an undefined value, you must specify it" on my callers. I'm a Perl programmer, so my programs are programmer friendly. Absence of named parameters are fine. I don't balk at extra arguments either - this can be very useful, as wrappers can just call the function with whatever @_ they were called with.

On the other hand, I might work with defaults. Then I'd write it as:

  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';
      ...
}
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Or:

  sub convert {
      my %arg = (from   => 'default',
                 to     => 'default',
                 thing  => 'default',
                 @_);
      my ($from, $to, $thing) = @arg{qw /from to thing/};
      ...
   }
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Or when passing in a false value doesn't make sense, I'd make it that any false value means 'default':

  sub convert {
      my %arg   = @_;
      my $from  = $arg{from}  || 'default';
      my $to    = $arg{to}    || 'default';
      my $thing = $arg{thing} || 'default';
      ...
   }
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And when I want to do something fancy, I use Getopt::Long:

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