So what you want is to print out the subscript separator $;, but not print it out if it is completely blank (i.e. just whitespace, empty string), or if it is undefined?

If I understand you right you want:

if ( defined $operator and $operator =~ /\S/ ) {
  #print operator here
  #it is defined and contains something 
  #that is not whitespace
}
else { 
  #do something else as $operator is either
  #not defined or it does not contain a
  #non whitespace character
}
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If I misinterpretted your intention let us know.

-enlil