# open new output file ">"
open (OUTPUTFILE, ">mysubdir/myoutputfile.txt") || die; 
	print OUTPUTFILE "my line 1\n";
	print OUTPUTFILE "my line 2\n";
	print OUTPUTFILE "my line 3\n";
close OUTPUTFILE; 

# open existing output file and append ">>"
open (OUTPUTFILE, ">>mysubdir/myoutputfile.txt") || die; 
	print OUTPUTFILE "my line 4\n";
	print OUTPUTFILE "my line 5\n";
close OUTPUTFILE;