use warnings;
use strict;

my $str = 'Iteration {Applied Field}  {Total Energy}    Mx';

while(length($str)>0){
	#print "str:$str:\n";
	if($str =~ m/^({([\s\w:]+)}(\s+)?)/){
		print "1: $2\n";
		$str =~ s/^$1//;
	}
	elsif($str =~ m/^(([\w:]+)(\s+)?)/){
		print "2: $1\n";
		$str =~ s/^$1//;
	}
	else{die "error";}
}

Result:
2: Iteration 
1: Applied Field
1: Total Energy
2: Mx