I believe I have addressed your concerns with this:

[%# Run this through tpage - from Template %]

[% debug = 0 %]

[% numbers = [ 1, 2, 3, 4, 5, 6 ] -%]
[%   end   = '\)' -%]
[%   start = '\w+\(' -%]


my %functions = ();


[% FOR level = numbers -%]
$functions{ [% level %] } = sub { 
  my $string = shift;

  [% IF debug %]my $warn = "running [% level %] on '$string'";[% END %
+]

  $string =~ s/[% end %]{[% level %]}$//;  
  $string =~ s/^([% start %]){[% level %]}//;

  [% IF debug %]warn "$warn to produce '$string'\n\n";[% END %]
  return $string;
};
[% END -%]


$functions{-1} = sub {
  my $string = shift;
  $string =~ m/\(([^()]+)\)+/;

  [% IF debug %]warn "running -1 on $string to produce '$1'\n";[% END 
+%]

  return $1;
};

sub get_proparg_evdb {
 my $string = shift;
 my $level  = shift || return $string;

 return &{$functions{ $level }}( $string ) 
   || die 'Add universal function here';
}
[download]

I also wrote the following test code which should check that the function is behaving as it should:

# Test the function against another with known good results.
for (@data) {
  my @args = split;

  my @copy = @args;

  my $A = get_proparg( @args );
  my $B = get_proparg_evdb( @args );

  unless ( $A eq $B ) {

    warn "function wrong for '$args[1]'\n";
    warn "A is '$A'\nB is '$B'\n\n";

  }

  warn "array messed up\n" unless join(' ',@copy) eq join(' ',@args);

}
[download]

I took into account the uselessness of \o (thank you diotalevi) and also removed the case == -1 check by creating a function for it. Interestingly the results are now even better:

            Rate m-elegant       ori     ori++      evdb
m-elegant 1797/s        --       -6%      -11%      -36%
ori       1914/s        7%        --       -5%      -32%
ori++     2016/s       12%        5%        --      -28%
evdb      2818/s       57%       47%       40%        --
[download]

If I have misunderstood your critique please let me know. As you can see you can set debug = 1 in the template stuff to enable messages during the run that shows that all cases are being used.

--tidiness is the memory loss of environmental mnemonics