Well the answer depends on how you get the value from the "box". Most UI components return empty input as an empty string, but some might return undef instead.

# let's say you've got the user input in $input..

if (defined($input) && $input ne "") {
  # input is defined and not empty
}
else {
  print "input wasn't filled in\n";
}
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Note that compare strings with eq and ne instead of == and != since == and != coerce their arguments to numbers.

See equality operators and defined in the documentation.