Sometimes it's better to extract out what you want rather than trying to split out what you don't want. (\K available with Perl 5.10+.)

>perl -wMstrict -le
"my $str = 'Iteration {Applied Field}  {Total Energy}  {Foo}  {  a b  
+ c d  }  Mx';
 ;;
 my $rx_word  = qr{ [[:alnum:]]+ }xms;
 my $rx_curly = qr{ { \s* \K  $rx_word (?: \s+ $rx_word)*  (?= \s* }) 
+}xms;
 ;;
 my @fields = $str =~ m{ $rx_curly | $rx_word }xmsg;
 printf qq{'$_' } for @fields;
"
'Iteration' 'Applied Field' 'Total Energy' 'Foo' 'a b   c d' 'Mx'
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