#! usr/bin/perl -w
use strict; 

open (INPUT, "proteins.txt") or die "Couldn't open proteins.txt for reading: $! \n"; 
open (OUTPUT, ">reverse_decoy.txt") or die "Couldn't open reverse_decoy.txt for writing: $! \n";

my @protein = <INPUT>; 
print OUTPUT "@protein \n"; 

foreach my $protein (@protein) {
	if ($protein =~ s/^>(.*\n)/> REVERSE DECOY $1/g) { 
	my $header = $protein; 
	print OUTPUT "$header"; 
	}
	else { 
		my $sequence = $protein; 
		$sequence =~ s/\n//; 
		my @reversed_sequence = split(/\s+/, reverse($sequence));
		print OUTPUT "@reversed_sequence\n"; 
		}}

close INPUT; 
close OUTPUT;