First, get the filename from the command line (using the Getopt::Std or Getopt::Long module, perhaps.)

# Redirect STDOUT.

if ($filename) {
   open(STDOUT, '>', $filename)
   or die("Can't open redirect STDOUT: $!\n");
}

print(...);  # Goes to file if one was specified, else STDOUT.
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-or-

# Use select.

if ($filename) {
   open(FILE, '>', $filename)
   or die("Can't open $filename: $!\n");
   select(FILE);
}

print(...);  # Goes to file if one was specified, else STDOUT.
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-or-

# If you want to output to STDOUT and to the file,
# create a handle which may point to STDOUT.

if ($filename) {
   open(FILE, '>', $filename)
   or die("Can't open $filename: $!\n");
} else {
   open(FILE, ">&STDOUT");
   or die("Can't dup STDOUT: $!\n");
}

print FILE (...);  # Goes to file if one was specified, else STDOUT.
print(...);        # Goes to STDOUT.
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