use IO::Handle;
my $infile= "input.txt";
my $outfile= "sorted.txt";
my $in= IO::Handle->new();
open $in, "< $infile\0"
    or  die "Can't read $infile: $!\n";
my $out= IO::Handle->new();
open $out, "> $outfile\0"
    or  die "Can't write $outfile: $!\n";
my @list= <$in>;
close $in;
# ... sort code here ...
print $out @list;