my $empty = $opt->{empty} || '';
my $trim_empty = !$empty;
my $keep_empty = $empty eq 'fill';
my $empty_replacement = $empty eq 'blank' ? '' : undef;
...
if ( length($line) || $keep_empty ) {
  push @array, $prefix . $line . $suffix;
}
elsif ( !$trim_empty ) {
  push @array, $empty_replacement;
}