$ cat xx.pl
#!/usr/local/bin/perl -w

use strict;

my $file = shift or die "File name required\n";

open STDIN, "<$file" or die "Can't open STDIN from $file - $!\n";
my @data = <STDIN>;
close STDIN or die "Error closing stdin - $!\n";
open STDOUT, "> $file" or die "Can't open STDOUT to $file - $!\n";
foreach (@data) {  print lc($_); }
close STDOUT or die "Error closing stdout - $!\n";

__DATA__

$ xx.pl abc
Filehandle STDIN reopened as STDOUT only for output at ~/mytest/tbin/xx.pl line 16.