You should not fear no warnings 'uninitialized'.

It's not generally a good idea to set it globally but when you really want undef to behave like an empty string for a few lines it's not a bad approach.

{
  no warnings 'uninitialized';
  print $op if $op =~ /\S/;
}
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To be honest it's probably not appropriate for a single line like this but it's a good tool to have in your box.

Another useful tool is for(SCALAR).

for ($op) {
  print if defined && /\S/;
}
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