Basic I/O Operations...

open (IN, $infile);
open (OUT,">$outfile"); # Write/Replace
open (OUT, ">>$outfile"); # Write/Append
open (SORTED, "|/usr/bin/sort > $sortfile"); # pipe to external process
 
my $line;
while ($line = <IN>) { 
   print OUT $line;
   print SORTED $line;
}
close IN;
close OUT;
close SORTED;

I recommend the FileHandle type to the Perl glob file handles. They can handle recursion better without naming conflicts! Note the object-oriented syntax

use FileHandle;
my $in = new FileHandle($infile); # Read
my $out = new FileHandle(">$outfile"); # Write/Replace
my $out = new FileHanlde(">>$outfile"); # Write/Append
my $sorted = new FileHandle("|/usr/bin/sort > $sortfile"); # pipe to external process
 
my $line;
while ($line = <$in>) { 
   $out->print($line);
   $sorted->print($line);
}
$in->close();
$out->close();
$sorted->close();