How to read from a file and direct output to a file if a file name is given in the command line, and printing to console if no argument given - perl

Question

I made a file, "rootfile", that contains paths to certain files and the perl program mymd5.perl gets the md5sum for each file and prints it in a certain order. How do I redirect the output to a file if a name is given in the command line? For instance if I do
perl mymd5.perl md5file
then it will feed output to md5file. And if I just do
perl mydm5.perl
it will just print to the console.
This is my rootfile:
/usr/local/courses/cs3423/assign8/cmdscan.c
/usr/local/courses/cs3423/assign8/driver.c
/usr/local/courses/cs3423/assign1/xpostitplus-2.3-3.diff.gz
This is my program right now:
open($in, "rootfile") or die "Can't open rootfile: $!";
$flag = 0;
if ($ARGV[0]){
open($out,$ARGV[0]) or die "Can't open $ARGV[0]: $!";
$flag = 1;
}
if ($flag == 1) {
select $out;
}
while ($line = <$in>) {
$md5line = `md5sum $line`;
#md5arr = split(" ",$md5line);
if ($flag == 0) {
printf("%s\t%s\n",$md5arr[1],$md5arr[0]);
}
}
close($out);

Answer 1

If you don't give a FILEHANDLE to print or printf, the output will go to STDOUT (the console).
There are several way you can redirect the output of your print statements.
select $out; #everything you print after this line will go the file specified by the filehandle $out.
... #your print statements come here.
close $out; #close connection when done to avoid counfusing the rest of the program.
#or you can use the filehandle right after the print statement as in:
print $out "Hello World!\n";

Answer 2

You can print a filename influenced by the value in #ARGV as follows:
This will take the name of the file in $ARGV[0] and use it to name a new file, edit.$ARGV[0]
#!/usr/bin/perl
use warnings;
use strict;
my $file = $ARGV[0];
open my $input, '<', $file or die $!;
my $editedfile = "edit.$file";
open my $name_change, '>', $editedfile or die $!;
if ($input eq "md5file"){
while ($in){
# Do something...
print $name_change "$_\n";
}
}

Answer 3

Perhaps the following will be helpful:
use strict;
use warnings;
while (<>) {
my $md5line = `md5sum $_`;
my #md5arr = split( " ", $md5line );
printf( "%s\t%s\n", $md5arr[1], $md5arr[0] );
}
Usage: perl mydm5.pl rootfile [>md5file]
The last, optional parameter will direct output to the file md5file; if absent, the results are printed to the console.