Your best bet is to put input into a variable, and test from here. That avoids icky little "but I didn't mean to read twice!" bugs. Also, direct comparisons are going to be problematic unless you remove the trailing newline that comes with the input. E.g.,

  my $input = <>;
  chomp($input);   # remove trailing \n
  if ( $input eq "" ) { ... }
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But none of that helps when you need to be looking at @ARGV instead.