use strict;

my ($filename_in, $filename_out);

while ($filename_in eq "") {
  print "IN file: ";
  $filename_in = <>;
  chomp $filename_in;
}

while ($filename_out eq "") {
  print "OUT file: ";
  $filename_out = <>;
  chomp $filename_out;
}

print "IN:  [$filename_in]\n";
print "OUT: [$filename_out]\n";