Where and with what is  $_ being assigned? If  $_ does not match the pattern, the while loop will never be entered. If  $_ does match, the while loop will be infinite.

c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = 'no joy';
 ;;
 my $output = '';
 while($_ =~ /\A([S\-\+XIO]+)\z/g)
 {
     my $part = $1;
     $output = $output . $part;
 }
 print qq{final output: '$output'};
"
final output: ''
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Please see Short, Self-Contained, Correct Example.